Nevermind. I fixed it. ChatGPT saves the day. However, I did reboot test to see if it would happen again, and it did. Only this time, it was a different window that was lost. I used a file I made earlier to restore it again. But it looks like Firefox won't save more than 10 windows when closing for some reason. So now I need help resolving this problem.

Edit: The "about:config" setting "browser.sessionstore.max_windows_undo" was the culprit. It was set to 10, but I raised it to 50, and did another reboot test, and no lost windows this time. On the off chance that someone else encounters this same problem, I hope this helps.

The default value of that preference is 3, so most people would have been much worse off. To close Firefox safely -- so Firefox doesn't mark individual windows as closed -- use the menu. Either:

• menu button > Exit
• (menu bar or Alt+F) File > Exit

If the tabs in the missing window were critical and you can't recover them from history because they were open for a while, you could try extracting the file Firefox created at the last automatic update. Here's how:

(1) To open your profile folder...

You can open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > Help > More Troubleshooting Information
• (menu bar or Alt+H) Help > More Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, find the Profile Folder row and click the "Open Folder" button. This should launch Windows' File Explorer listing the contents of your current profile folder.

(2) Copy out session history files

In your profile folder, double-click into the sessionstore-backups folder. Save files from the past two weeks to a safe location such as your Documents folder.

(3) What files did you find?

The kinds of files you may find among your sessionstore files are:

• recovery.jsonlz4: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.baklz4: a backup copy of recovery.jsonlz4
• previous.jsonlz4: the windows and tabs in your last Firefox session {possibly too late for this file}
• upgrade.jsonlz4-build_id: the windows and tabs in the Firefox session that was live at the time of your last update {try the most recent one of these}

Optional File Contents Preview

These compressed files are a pain to view, so I created a tool on my website to list out their contents. If you want to try that, you can drag and drop it onto the large box on the following page, then click the "Scrounge URLs" button:

https://www.jeffersonscher.com/ffu/scrounger.html

If you don't get a list within 15 seconds, that probably means the script is caught in a loop. You may need to close the tab to avoid a tab crash and then try again in a new tab.

If you get a useful list, use the "Save List" button to archive it as a web page of clickable links for future reference, in case no other approach is successful.