Firefox goes by the Content Type (also called the MIME or media type) of a downloaded file when opening files. See Manage file types and download actions in Firefox.

Can to post links to some of the files that are opening in the wrong application?

A Linux user might be the better person for answering your questions but a screenshot of your Firefox Applications settings might also be helpful, showing the Content Type and Action for image files and what application is set to open for those content types (you can filter the list from the Applications Search bar).

If you are opening a file from your own folders, that is an OS or DE matter. I have to set applications for each image file type. I have attached a screenshot which shows only part of the list.

https://user-media-prod-cdn.itsre-sumo.mozilla.net/uploads/images/2022-03-14-02-18-53-ad921d.png On my OS, the default application is gwenview, the first on the list, its the default application for basically every image file type.

https://user-media-prod-cdn.itsre-sumo.mozilla.net/uploads/images/2022-03-14-02-34-46-d82493.png

when i click to open the image from firefox download manager, it opens the image with gimp instead of gwenview, that is configured in both system and firefox

When you click on a download link for a JPEG Image in Firefox, such as one of the download links on this test page, https://sample-videos.com/download-sample-jpg-image.php ...it should open in the gwenview application you have set to open JPEG downloads in the Applications section of Firefox settings.

It works for me on Windows, when I choose the Applications "Use Other..." option for downloaded JPEG Image files to use IrfanView instead of the default Windows Photo Viewer application. If it doesn't work for you, it might be a Linux issue and you may want to file a Firefox "File Handling" bug report on https://bugzilla.mozilla.org or wait fro more help from a Linux user.

Just to clarify: You wrote, "when i click to open the image from firefox download manager, it opens the image with gimp instead of gwenview,..." Can you explain what you mean by the Firefox "download manager"?

Do you mean the Download panel, ... which looks like this? ... or the Downloads Library window, which looks like this?

Or, are you opening the files from the folder on your computer that contains the downloaded files? (Clicking the folder icon to the right of the file entry opens the containing folder on your computer).

If you are you opening the JPEG Image files from the folder on your computer where downloads are saved, then it will open in your system default application that opens JPEG images.

The article Where to find and manage downloaded files in Firefox has more information on these locations.

P.S. In case it helps, found this old, still open "Firefox File Handling" Linux bug that seems related:

Bug 1304650 Firefox use wrong default applications to open files in KDE5 (quote from Description) It opens all images with GIMP, rather than default image viewer.

When you click on a download link for a JPEG Image in Firefox, such as one of the download links on this test page, https://sample-videos.com/download-sample-jpg-image.php ...it should open in the gwenview application you have set to open JPEG downloads in the Applications section of Firefox settings. It works for me on Windows, when I choose the Applications "Use Other..." option for downloaded JPEG Image files to use IrfanView instead of the default Windows Photo Viewer application. If it doesn't work for you, it might be a Linux issue and you may want to file a Firefox "File Handling" bug report on https://bugzilla.mozilla.org or wait fro more help from a Linux user. Just to clarify: You wrote, "when i click to open the image from firefox download manager, it opens the image with gimp instead of gwenview,..." Can you explain what you mean by the Firefox "download manager"? Do you mean the Download panel, ... which looks like this? ... or the Downloads Library window, which looks like this? Or, are you opening the files from the folder on your computer that contains the downloaded files? (Clicking the folder icon to the right of the file entry opens the containing folder on your computer). If you are you opening the JPEG Image files from the folder on your computer where downloads are saved, then it will open in your system default application that opens JPEG images. The article Where to find and manage downloaded files in Firefox has more information on these locations.

Both the download tab and download history window I'll report this bug now, i haven't found anyone with the same problem with mine, i believe it's a specific linux bug, thanks for helping me everyone