I am now trying to disable this new feature by setting this pref to false on the about:config page.

   browser.helperApps.showOpenOptionForPdfJS = false 

I'll report back....

Curious... this seemed to slow things down... but did not take care of the problem. I'm still fighting to control which app opens my pdf files, and Firefox continues to override my choice... because this About:Config setting itself was changed from how I set it! (Could that be a Sync issue?)

Can anyone assist, please?

Try to rename/remove handlers.json in the Firefox profile folder with Firefox closed to reset all file actions.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Cor-El... thanks. I deleted the handlers.json file, and upon launch Firefox recreated it, as expected.

The problem then re-occurred. So I have gone into preferences to check that I specified in Firefox how to handle pdf's, as well as in Windows, and I see that for now my preferences are set for Adobe Acrobat Pro to open pdf files. Perhaps I needed to take that additional step once again... so now, I am in the waiting/testing phase to see if this is able to hold. I'll report back after a while.

A question... I have several computers running Firefox that are synced... so I am aware of at least three Windows PCs plus my Android phone that have been operational as this goes on, and as best I can tell settings from one are not messing up another... but would it be possible that I'd have to simultaneously try this fix on all three computers at once (or at least check them all, like in the attached screenshot), to stop unwanted preferences from bleeding into other systems?

Thanks....

Well, that didn't take long. Within 10 minutes of attempting the suggested change, my preferences got reset to Firefox opening pdf's instead of Adobe Acrobat Pro. More details shortly.

Further confirmed in a weird way... while downloading attached pdf's from a webmail client, Firefox now remembers my preference to open in Adobe Acrobat Pro--which is good--yet it has nevertheless changed the Win10 file association preference to Firefox, so any pdf file on my drive is now associated with Firefox and not Adobe Acrobat. I change it back, and my choice lasts for a little while.

I've not yet been able to conclusively observe what event or timing triggers the the change in Win10 pdf setting back to Firefox. It has seemed to be more a matter of time than some action, but I've been working and not trying to carefully watch pdf files, so I don't have data yet.

Update on 12/16/2020: Behavior of Win10's preferences being reset to Firefox as preferred pdf app continues. Still have not found trigger... happens between every day or so, to several times a day. Will try to continue to track workflow to see if I can identify specific actions that lead to Win10's preferences being reset.

Okay, I've verified when Win10 file preferences get rest... and while this sounds crazy, I'd be glad for help to fix it. Win10 file preferences for pdf files get reset to Firefox whenever "Save As" within Adobe Acrobat Pro is chosen for a pdf file that has been directly opened from Firefox with my Adobe Acrobat Pro.

Specifically, preferences are *not* reset when a file is opened or downloaded (if a pdf is only viewed, and not saved, preferences do not get reset). And, specifically, not when a file is actually saved... but when the act of saving is begun with "Save As."

Weird, huh? Who can assist me on this?

Enten

That is truly bizarre.

Your Options page screenshot indicates a very old version of Acrobat, so it might be difficult to research weird new interactions with the Windows setting in the era of Firefox 78+ (where Firefox takes a larger role in handling PDFs).

As a workaround, I wonder whether you could set up a shortcut that would restore Acrobat as the default handler using a batch file or PowerShell script, etc.

As a workaround, I wonder whether you could set up a shortcut that would restore Acrobat as the default handler using a batch file or PowerShell script, etc.

Thanks for responding! And yes... my version of Acrobat Pro is old... but still does what it needs to do.

But an interesting thought about a batch file or script... I don't know how to do that with preferences. [Anyone have a hint?] --So, to reset I currently right-click on any pdf file, choose "Open with," ignore the options and select "Choose another app," then select Acrobat Pro and check the box to always use that app... doesn't take too long, but double-clicking a script would sure be easier.

Or... hadn't thought about this until now... perhaps I should uninstall Acrobat Pro, and reinstall it? Not a simple process anymore, but I wonder...... hmm....