Seriously. Mine is populated.

Yes. But I am suggesting is that if I change "Preview in TB" to "Always ask" then the submenu on "Open" show "Preview in TB" as an option. As it is, the choice is no preview and choice of opening app, or preview and no choice.

Testing: I just changed my Preferences for pdf to 'Always Ask'

When I select to open pdf attachment a window opens asking what to do. I select 'Open With' and click on drop down which offers 'Adobe Acrobat' and 'Other'. I click on 'Other' and get a larger list which includes Thunderbird.

If I select 'Thunderbird' then it opens a Write window and the pdf is shown as an attachment. Obviously this is not desired.

If I select to open again - I now see 'Adobe Acrobat', 'Thunderbird' and 'Other' 'Other' offers quite a list including 'Firefox' This does open the pdf in a new Firefox tab. OK, I can read the pdf.

If I select to open again - I now see 'Adobe Acrobat', 'Firefox' and 'Other' So at least the list gets populated with the default - adobe and also the last selected option.

You would expect the 'Thunderbird' option to do the same as the 'Firefox' option - open pdf to read in a new tab in Thunderbird, but it does not and it does not offer 'Preview in Thunderbird'. This behaviour is not really expected, so I'll check to see if this has previously been reported as a bug.

Located a bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1734900

Thanks. I did not think I was the only one. So perhaps some future update will do something about it.

Last week, I could pull up a pdf document in preview. I would open it in Adobe use fill and sign and save the document in a special folder. I would upload it back to the server and check preview to make sure that I uploaded the correct file.

It was easy to keep track of the number of documents that I processed and the workflow was easy.

This week, I can't preview things in Firefox, I either preview and open in Firefox or open in Adobe. Files don't just open in Adobe, they save to the current folder, so now I have two files and counting. And when I upload the file I can't just click on the preview button to make sure that it's correct, the file downloads and saves again and again and again. It's very hard to keep track of which file I've uploaded when the system keeps saving files right and left and really difficult to count how many files that I've processed.

How do I stop this?

Last week, I could pull up a pdf document in preview. I would open it in Adobe use fill and sign and save the document in a special folder..... This week, I can't preview things in Firefox, I either preview and open in Firefox or open in Adobe. Files don't just open in Adobe, they save to the current folder, so now I have two files and counting. And when I upload the file I can't just click on the preview button to make sure that it's correct, the file downloads and saves again and again and again.

You need to tell us: What version of Thunderbird are you using? If you are not using the latest version : 91.7.0 then update Thunderbird to see if this fixes the situation.

What settings you have got set up preferences ? Menu app icon > Preferences > General scroll to 'Files & Attachments' section Content type: Portable Document Format (PDF) What is set up as the 'Action' ? Click on drop down and choose 'Use Adobe ' option.