%APPDATA%\Mozilla\Firefox\Profiles

Is your Firefox asking at startup whether you want to restore a crashed session, or are you using the History menu to restore the old session? '''Back up Session History files''' Firefox creates numerous session history files, but because session history is only for the immediately previous session, it is unfortunately too easy to lose it. Could you make a backup copy of your existing session history files? Here's how: '''(1) To open your profile folder...''' ''If Firefox is still running:'' You can open your current Firefox settings (AKA Firefox profile) folder using either * "3-bar" menu button > Help > More Troubleshooting Information * (menu bar) 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 on that row (the other ones don't help). This should launch Windows' File Explorer listing the contents of your current profile folder. ''If Firefox is closed:'' Type or paste the following into the Windows Run dialog or the system search box and press Enter to launch Windows Explorer: %APPDATA%\Mozilla\Firefox\Profiles In that folder, do you see a semi-randomly-named folder? If so, click into it. If you find multiple such folders, find the one that was most recently updated. '''(2) Copy out session history files''' In your profile folder, double-click into the '''sessionstore-backups''' folder. Save all files here to a safe location such as your Documents folder. (You can select all files using Ctrl+A, copy using Ctrl+C, then open the other folder and paste using Ctrl+V or of course the right-click menu has these as well.) '''(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 * upgrade.jsonlz4-''build_id'': the windows and tabs in the Firefox session that was live at the time of your last update Sometimes you can tell from the last modification time which one will have your missing tabs, but not always. '''''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.