Hello,

I am sure you know this, but to be sure, it is normal that you can not get the tabs back. Private windows avoid writing to the history file (as well as a few others), meaning when they are closed, the tabs that were open are nearly impossible to recover.

Do the normal windows also unexpectedly closing? If so, it may be a problem with an extension or another program. If not, I would still like you to try troubleshoot mode (previously known as safe mode).

Instructions below

Try Firefox Troubleshoot Mode to see if the problem goes away. Firefox Troubleshoot Mode (previously known as Firefox Safe Mode) is a special diagnostic mode that temporarily turns off hardware acceleration, disables add-ons (extensions and themes) and resets some settings. See the Diagnose Firefox issues using Troubleshoot Mode article for details.

If Firefox is open, you can restart in Firefox Troubleshoot Mode from the Help menu:

• Click the menu button , click Help, select Troubleshoot Mode and click Restart in the "Restart Firefox in Troubleshoot Mode?" dialog that opens.

If Firefox is not running, you can start Firefox in Troubleshoot Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Troubleshoot Mode window appears, select "Open".

If the issue is not present in Firefox Troubleshoot Mode, your problem is probably caused by an extension, theme or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Troubleshoot Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again. (If you can't exit Firefox Troubleshoot Mode, please see Firefox is stuck in Troubleshoot Mode).

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

Firefox doesn't show any warnings when you close private windows that would stop the action, and you can't recover a closed private window, so it's a serious problem.

Many years ago I created a rudimentary add-on that opens a "Keep Open" tab in a new window. You can activate protection in the tab and when the window is closing, a script in the tab will trigger a message asking if you really want to navigate away from the page. Clicking "Stay on page" will keep the window open. (You may have seen this somewhat annoying message on a website from time to time.)

That approach can keep your window from closing while you figure out what is causing the problem.

https://addons.mozilla.org/firefox/addon/prevent-closing-window/

(Sorry for the clunky design, I'm not a professional designer or programmer.)

Thank you for your responses!

I only have 1 extension that I keep running (OneTab), and I've been using it for years now. I keep all other extensions disabled until I need them.

I will use the extension while I figure out what the issue is.

Thanks again.