Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/kb/safe-mode

Does the problem go away?

Nope, the problem remains the same.

Please confirm you did start Windows in safe mode. Please confirm you did start Thunderbird in safe mode.

I did start Windows in safe mode. I did start Thunderbird in safe mode.

Please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information. Press the "Copy text to clipboard button" and paste the information into your reply.

Nom: Thunderbird

   Version: 60.2.1
   Identifiant de compilation: 20180930223627

   Canal de mise à jour: release
   Agent utilisateur: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1
   Système d’exploitation: Windows_NT 10.0

   Fenêtres multiprocessus: 0/0 (Désactivé)
   Processus de contenu web:
   Stylo: content = true (activé par défaut), chrome = true (activé par défaut)
   Stratégies d’entreprise:
   Clé Google: Manquante
   Clé du service de localisation de Mozilla: Manquante
   Mode sans échec: false

 Comptes courrier et groupes
   account1:
     INCOMING: account1, , (imap) imap.gmail.com:993, SSL, OAuth2
     OUTGOING: , smtp.gmail.com:465, SSL, OAuth2, true

   account4:
     INCOMING: account4, , (imap) zimbra.u-psud.fr:993, SSL, passwordCleartext
     OUTGOING: , smtps.u-psud.fr:465, SSL, passwordCleartext, true

   account5:
     INCOMING: account5, , (imap) imap.laposte.net:993, SSL, passwordCleartext
     OUTGOING: , smtp.laposte.net:465, SSL, passwordCleartext, true

 Rapports de plantage des 3 derniers jours

 Tous les rapports de plantage

 Extensions

     Nom
     Version
     Activée
     ID

     Lightning
     6.2.2.1
     false
     {e2fda1a4-762b-4020-b5ad-a41df1933103}

 Logiciel de sécurité

   Type: Nom

     Antivirus: Antivirus Windows Defender
     Logiciel anti-espion: Antivirus Windows Defender
     Pare-feu: Pare-feu Windows

 Accélération graphique

     Fonctionnalités
     Composition: Basic
     Zoom/Panoramique asynchrones: aucun
     Pilote WebGL 1 - Informations WSI: EGL_VENDOR: Google Inc. (adapter LUID: 0000000000008e71)

EGL_VERSION: 1.4 (ANGLE 2.1.0.7edc21933ef1) EGL_EXTENSIONS: EGL_EXT_create_context_robustness EGL_ANGLE_d3d_share_handle_client_buffer EGL_ANGLE_d3d_texture_client_buffer EGL_ANGLE_surface_d3d_texture_2d_share_handle EGL_ANGLE_query_surface_pointer EGL_ANGLE_window_fixed_size EGL_ANGLE_keyed_mutex EGL_ANGLE_surface_orientation EGL_ANGLE_direct_composition EGL_NV_post_sub_buffer EGL_KHR_create_context EGL_EXT_device_query EGL_KHR_image EGL_KHR_image_base EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_gl_renderbuffer_image EGL_KHR_get_all_proc_addresses EGL_KHR_stream EGL_KHR_stream_consumer_gltexture EGL_NV_stream_consumer_gltexture_yuv EGL_ANGLE_flexible_surface_compatibility EGL_ANGLE_stream_producer_d3d_texture EGL_ANGLE_create_context_webgl_compatibility EGL_CHROMIUM_create_context_bind_generates_resource EGL_CHROMIUM_sync_control EGL_EXT_pixel_format_float EGL_KHR_surfaceless_context EGL_ANGLE_display_texture_share_group EGL_ANGLE_create_context_client_arrays EGL_ANGLE_program_cache_control EGL_ANGLE_robust_resource_initialization EGL_EXTENSIONS(nullptr): EGL_EXT_client_extensions EGL_EXT_platform_base EGL_EXT_platform_device EGL_ANGLE_platform_angle EGL_ANGLE_platform_angle_d3d EGL_ANGLE_device_creation EGL_ANGLE_device_creation_d3d11 EGL_ANGLE_experimental_present_path EGL_KHR_client_get_all_proc_addresses

     Pilote WebGL 1 - Rendu: Google Inc. -- ANGLE (NVIDIA GeForce GTX 1060 6GB Direct3D11 vs_5_0 ps_5_0)
     Pilote WebGL 1 - Version: OpenGL ES 2.0 (ANGLE 2.1.0.7edc21933ef1)
     Pilote WebGL 1 - Extensions: GL_ANGLE_client_arrays GL_ANGLE_depth_texture GL_ANGLE_framebuffer_blit GL_ANGLE_framebuffer_multisample GL_ANGLE_instanced_arrays GL_ANGLE_lossy_etc_decode GL_ANGLE_pack_reverse_row_order GL_ANGLE_program_cache_control GL_ANGLE_request_extension GL_ANGLE_robust_client_memory GL_ANGLE_texture_compression_dxt3 GL_ANGLE_texture_compression_dxt5 GL_ANGLE_texture_usage GL_ANGLE_translated_shader_source GL_CHROMIUM_bind_generates_resource GL_CHROMIUM_bind_uniform_location GL_CHROMIUM_color_buffer_float_rgb GL_CHROMIUM_color_buffer_float_rgba GL_CHROMIUM_copy_compressed_texture GL_CHROMIUM_copy_texture GL_CHROMIUM_sync_query GL_EXT_blend_minmax GL_EXT_color_buffer_half_float GL_EXT_debug_marker GL_EXT_discard_framebuffer GL_EXT_disjoint_timer_query GL_EXT_draw_buffers GL_EXT_frag_depth GL_EXT_map_buffer_range GL_EXT_occlusion_query_boolean GL_EXT_read_format_bgra GL_EXT_robustness GL_EXT_sRGB GL_EXT_shader_texture_lod GL_EXT_texture_compression_dxt1 GL_EXT_texture_compression_s3tc_srgb GL_EXT_texture_filter_anisotropic GL_EXT_texture_format_BGRA8888 GL_EXT_texture_rg GL_EXT_texture_storage GL_EXT_unpack_subimage GL_KHR_debug GL_NV_EGL_stream_consumer_external GL_NV_fence GL_NV_pack_subimage GL_NV_pixel_buffer_object GL_OES_EGL_image GL_OES_EGL_image_external GL_OES_compressed_ETC1_RGB8_texture GL_OES_depth32 GL_OES_element_index_uint GL_OES_get_program_binary GL_OES_mapbuffer GL_OES_packed_depth_stencil GL_OES_rgb8_rgba8 GL_OES_standard_derivatives GL_OES_surfaceless_context GL_OES_texture_float GL_OES_texture_float_linear GL_OES_texture_half_float GL_OES_texture_half_float_linear GL_OES_texture_npot GL_OES_vertex_array_object
     WebGL 1 - Extensions: ANGLE_instanced_arrays EXT_blend_minmax EXT_color_buffer_half_float EXT_frag_depth EXT_sRGB EXT_shader_texture_lod EXT_texture_filter_anisotropic EXT_disjoint_timer_query OES_element_index_uint OES_standard_derivatives OES_texture_float OES_texture_float_linear OES_texture_half_float OES_texture_half_float_linear OES_vertex_array_object WEBGL_color_buffer_float WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_depth_texture WEBGL_draw_buffers WEBGL_lose_context
     Pilote WebGL 2 - Informations WSI: EGL_VENDOR: Google Inc. (adapter LUID: 0000000000008e71)

EGL_VERSION: 1.4 (ANGLE 2.1.0.7edc21933ef1) EGL_EXTENSIONS: EGL_EXT_create_context_robustness EGL_ANGLE_d3d_share_handle_client_buffer EGL_ANGLE_d3d_texture_client_buffer EGL_ANGLE_surface_d3d_texture_2d_share_handle EGL_ANGLE_query_surface_pointer EGL_ANGLE_window_fixed_size EGL_ANGLE_keyed_mutex EGL_ANGLE_surface_orientation EGL_ANGLE_direct_composition EGL_NV_post_sub_buffer EGL_KHR_create_context EGL_EXT_device_query EGL_KHR_image EGL_KHR_image_base EGL_KHR_gl_texture_2D_image EGL_KHR_gl_texture_cubemap_image EGL_KHR_gl_renderbuffer_image EGL_KHR_get_all_proc_addresses EGL_KHR_stream EGL_KHR_stream_consumer_gltexture EGL_NV_stream_consumer_gltexture_yuv EGL_ANGLE_flexible_surface_compatibility EGL_ANGLE_stream_producer_d3d_texture EGL_ANGLE_create_context_webgl_compatibility EGL_CHROMIUM_create_context_bind_generates_resource EGL_CHROMIUM_sync_control EGL_EXT_pixel_format_float EGL_KHR_surfaceless_context EGL_ANGLE_display_texture_share_group EGL_ANGLE_create_context_client_arrays EGL_ANGLE_program_cache_control EGL_ANGLE_robust_resource_initialization EGL_EXTENSIONS(nullptr): EGL_EXT_client_extensions EGL_EXT_platform_base EGL_EXT_platform_device EGL_ANGLE_platform_angle EGL_ANGLE_platform_angle_d3d EGL_ANGLE_device_creation EGL_ANGLE_device_creation_d3d11 EGL_ANGLE_experimental_present_path EGL_KHR_client_get_all_proc_addresses

     Pilote WebGL 2 - Rendu: Google Inc. -- ANGLE (NVIDIA GeForce GTX 1060 6GB Direct3D11 vs_5_0 ps_5_0)
     Pilote WebGL 2 - Version: OpenGL ES 3.0 (ANGLE 2.1.0.7edc21933ef1)
     Pilote WebGL 2 - Extensions: GL_ANGLE_client_arrays GL_ANGLE_depth_texture GL_ANGLE_framebuffer_blit GL_ANGLE_framebuffer_multisample GL_ANGLE_instanced_arrays GL_ANGLE_lossy_etc_decode GL_ANGLE_multiview GL_ANGLE_pack_reverse_row_order GL_ANGLE_program_cache_control GL_ANGLE_request_extension GL_ANGLE_robust_client_memory GL_ANGLE_texture_compression_dxt3 GL_ANGLE_texture_compression_dxt5 GL_ANGLE_texture_usage GL_ANGLE_translated_shader_source GL_CHROMIUM_bind_generates_resource GL_CHROMIUM_bind_uniform_location GL_CHROMIUM_color_buffer_float_rgb GL_CHROMIUM_color_buffer_float_rgba GL_CHROMIUM_copy_compressed_texture GL_CHROMIUM_copy_texture GL_CHROMIUM_sync_query GL_EXT_blend_minmax GL_EXT_color_buffer_float GL_EXT_color_buffer_half_float GL_EXT_debug_marker GL_EXT_discard_framebuffer GL_EXT_disjoint_timer_query GL_EXT_draw_buffers GL_EXT_frag_depth GL_EXT_map_buffer_range GL_EXT_occlusion_query_boolean GL_EXT_read_format_bgra GL_EXT_robustness GL_EXT_sRGB GL_EXT_shader_texture_lod GL_EXT_texture_compression_dxt1 GL_EXT_texture_compression_s3tc_srgb GL_EXT_texture_filter_anisotropic GL_EXT_texture_format_BGRA8888 GL_EXT_texture_norm16 GL_EXT_texture_rg GL_EXT_texture_storage GL_EXT_unpack_subimage GL_KHR_debug GL_NV_EGL_stream_consumer_external GL_NV_fence GL_NV_pack_subimage GL_NV_pixel_buffer_object GL_OES_EGL_image GL_OES_EGL_image_external GL_OES_EGL_image_external_essl3 GL_OES_compressed_ETC1_RGB8_texture GL_OES_depth32 GL_OES_element_index_uint GL_OES_get_program_binary GL_OES_mapbuffer GL_OES_packed_depth_stencil GL_OES_rgb8_rgba8 GL_OES_standard_derivatives GL_OES_surfaceless_context GL_OES_texture_float GL_OES_texture_float_linear GL_OES_texture_half_float GL_OES_texture_half_float_linear GL_OES_texture_npot GL_OES_vertex_array_object
     WebGL 2 - Extensions: EXT_color_buffer_float EXT_texture_filter_anisotropic EXT_disjoint_timer_query OES_texture_float_linear WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_lose_context
     Painting en dehors du fil d’exécution principal activé: true
     DirectWrite: true (10.0.17134.1)

     GPU 1
     Actif: Oui
     Description: NVIDIA GeForce GTX 1060 6GB
     ID du vendeur: 0x10de
     ID du périphérique: 0x1c03
     Version du pilote: 23.21.13.9135
     Date du pilote: 3-23-2018
     Pilotes: C:\WINDOWS\System32\DriverStore\FileRepository\nv_ref_pubwu.inf_amd64_708ec8f9a4d134c6\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nv_ref_pubwu.inf_amd64_708ec8f9a4d134c6\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nv_ref_pubwu.inf_amd64_708ec8f9a4d134c6\nvldumdx.dll,C:\WINDOWS\System32\DriverStore\FileRepository\nv_ref_pubwu.inf_amd64_708ec8f9a4d134c6\nvldumdx.dll C:\WIN

As per your Troubleshooting Information there are three IMAP accounts. Which is the one showing the problem?

Avast is installed on my computer,

As per your Troubleshooting Information your anti-virus software is Windows Defender. Please clarify.

The last one is showing the problem.

I installed Avast. Windows Defender is installed by default with Windows 10.

I'm afraid I still don't really understand your problem.

Thunderbird plays sound notification when a new message is available

What do you mean with 'available'? The new message notification doesn't trigger when a new message is 'available' on the server. It is triggered when Thunderbird has received the message.

despite: - not showing the dialog notification

What dialog?

- not downloading the message itself -

How do you determine the message has not been downloaded, assuming there is a new message in the first place.

So basically are you saying there is some sort of sound notification, and you're assuming that this would indicate a new message has been received by Thunderbird? But then there is no evidence a new message has actually been received? Have you checked via webmail a new message is present on the server?

I installed Avast.

I do not doubt that. The question is which anti-virus software is actually active? There must only be one active real-time scanner, and Thunderbird indicates that this is Windows Defender.

Try to create an exception in Defender for the Thunderbird profile folder. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Thunderbird : - plays the sound of a new downloaded/"available" message - the email address becomes blue on the left side of the window (where all the accounts are listed)

However: - the "Reception folder" does not change its number of unread message (written in parenthesis) - there is no notification message on the lower right corner of my screen - there is no new message downloaded in the folder despite THERE IS ONE on the webmail interface

The only thing enabled in my Windows Defender configuration is the Firewall in which Thunderbird is allowed.

This all sounds very odd, and I'm not sure what else to suggest.

You can try to create a new profile using profile manager. https://support.mozilla.org/kb/using-multiple-profiles

Then start Thunderbird with the new profile also using profile manager.

Set up the troubled account in the new profile.

Do you still see the problem?

Yes, I still see the problem. I found other people on a French forum having the same issue discribed with laposte.net provider.

Then you should contact your email provider.

......

Thank you. For using Firefox.

Greatings