In Options/General/Files & Attachments, delete the entry for pdf content type, restart TB, double-click a pdf attachment, select the opening app by browsing to the app in Finder, don't select from the list, check the box 'Always open...'.

Unfortunately, that did not work. Still the same "no reaction" to double click or Open selection from menu for the attachment.

Can you say what happened since the last version of 78 was working? Do you have a security app that scans attachments?

No other changes have been made. No App that specifically scans attachments. I tried disconnecting my VPN but that had no effect - still no reaction to double click or selecting Open in the menu.

Tried disabling extensions I use (ImportExportTools NG 10.0.0, LookOut (fix versiion) 3.0.2 and ReplyWithHeader 2.3.0) but still no joy.

BTW, this behaviour is with all attachments, including ones that could be opened previously. And not just pdfs, but also .ics, .pptx, etc.

The first simple thing to try is to reset the file that defines the apps that open attachments. Help/Troubleshooting, Profile Folder, Show in Finder, close TB, rename or delete handlers.json, restart TB, open an attachment (if you can).

Renaming and moving handlers.json did not change behaviour. I also noted that no new replacement handlers.json file appears to have been created upon restarting TB and trying to open an attachment.

I have restored the handlers.json file back into its original folder.

Out of curiousity, is TB supposed to add newly encountered file types to handlers.json when they are opened as an attachment with the particular application selected to open the file? There is no longer an entry for pdf file type in handlers.json and pdf Content type is not listed in TB, Preferences, General, Files & Attachments. Is there a way to manually add a Content (file) type and Action to appear in the Files & Attachments area?

It would seem TB is no longer (1) using info in handlers.json, (2) updating info in handlers.json, (3) creating a new handlers.json if one is not found at start-up.

Additions to handlers.json are made when you open an attachment, specify the opening app, and check 'Always use this...'. It's not recommended to edit the file manually. But the problem here seems to be the inability to open any attachments. Does it work if you run in safe mode (hold Option when launching TB)? That would test for a broken layout.

No change running in safe mode. Other actions in Context / Save menu (i.e., Save As, Detach and Delete) appear to work properly in safe and regular mode. It is as if when Open is selected TB does not execute the "Open" functionality to ask for App to use, etc. - a no-op call.

I also cannot open files I have attached to Sent emails.

There's something wrong with the profile, and at the moment I can't see how to fix it. Create a new profile from Help/Troubleshooting, about:profiles, add an account, and see if the Open command works. This doesn't affect your current profile, and if the new profile works, mail and contacts etc. can be copied from the old to the new profile.

Sigh! Same behaviour with the new profile and existing gmail account added to it. Open and double click on attachments do nothing. I also note that there is no handlers.json file in the folder for the new profile even after attempting to open attachments. Not sure that profile is the issue - seems like the functionality is not there/not executing in the TB app.

Could I need to re-download and install v78.4 and can that be done without messing up current profile, email, calendar, etc.?

To rule out the obvious, can you open an attachment by selecting it then pressing Enter on the keyboard or numeric keyboard? The json file is empty until an opening app is defined and the 'Always..' box is checked.

You can reinstall TB without affecting your data in the profile folder(s).

Yes, attachments once saved (and all other files) can be opened by double click, or Enter key, in Finder or on Desktop.

I meant can you select attachments in the TB attachment pane and open them with Enter. There might be a clue in Tools/Developer Tools/Error Console.

Selecting the attachment and hitting Enter results in same behaviour. I have opened Tools/Developer Tools/Error Console but I am not able to interpret the contents. I did open the console upon starting TB and then watched to see if there was anything added as I tried the different ways to attempt to open the attachment but it did not appear that any messages were added.

Would contents of the error console be helpful? Is there a way to save it to a file, or is it available in a file that can be copied, to attach here? I hesitate to copy it into this window as it looks like information would be left behind?

The Error Console often shows a lot of irrelevant code, an since it didn't add anything when you tried to open a file, I don't think it's worth posting.

Since the problem exists in a clean, new profile, I can only suggest running OS X in safe mode in case there's some external app or driver issue causing it.

https://support.apple.com/en-us/HT201262

No change running in MacOS safe mode. I will try re-installing TB.

Problem solved! Found this issue also discussed back in 2018 wrt v60.

The problem and solution have a few components: 1. Permissions for my Desktop folder at some point were change to Read Only for me - perhaps during OS update? 2. Files & Attachments tab in Preferences although set to "Always ask me where to save files" also had default selection set to Desktop for the unchecked "Save files to"

Selecting and setting "Save files ..." to a folder with read-write permissions then re-selecting "Always ask ..." resulted in correct behaviour - attachments opened or window opened to ask what to do and app to use.

Modifying permissions on Desktop folder to read-write, selecting it as "Save files ..." then re-selecting "Always ask ..." resulted in correct behaviour.

From this I infer that Thunderbird first looks to the permissions on the folder identified in the "Save files ..." entry before using the selection of "Always ask ..." or "Save files ..." to determine behaviour. I presume this is due to a Thunderbird requirement for a place to put the file and then opening the file from that location in response the attachment Open action selected.

This ties in with previous observed behaviour before Desktop permission changed to read only that after opening and closing attachments from Thunderbird the Desktop was littered with the files that had been opened.

Now I have created a separate folder for "Save files ..." that I can clean up periodically and keep Desktop clean, and selected "Always ask ..." for occasions when I want to save attached files.

In hindsight the behaviour is not unreasonable but perhaps the Files & Attachments area in Thunderbird Preferences could be updated in a future release to provide this bit of information about the importance of the "Save files to" folder selection even if "Always ask ..." is the option selected.