%APPDATA%\Mozilla\Firefox\Profiles

Sorry to hear about this problem. I'm not sure which error you saw: did you get a screen at startup offering a choice between restoring some windows or starting a completely new session? ---- Usually Firefox will attempt an automatic crash recovery for a regular (non-private) session. Could you check whether either of these are available (not hidden or grayed out): * "3-bar" menu button > History > Restore Previous Session * (menu bar) History > Restore Previous Session If they are missing or grayed, check either: * "3-bar" menu button > History > Recently Closed Windows * "Library" toolbar button > History > Recently Closed Windows * (menu bar) History > Recently Closed Windows ---- ''If your session isn't available that way:'' 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 start by making a backup of your existing session history files? Here's how: ''Do not exit Firefox, or if you closed it, don't re-open it.'' '''(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. 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. '''(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. '''''Next Steps''''' You can swap some files to trick Firefox into using one of the older ones at startup. More details once you know which file you want to try.