Hi,

You can try to Restore previous session via History (Alt + S). This may not work if Firefox was closed more than one time after the tabs were lost. You can also check Recently Closed Tabs in the History menu and try to restore individually.

Session Restore

Tab Groups

App Tabs

Thank you for trying. Unfortunately, I lost all my tabs including two that I was going to look at later and now cannot find them or identify them. How can I stop this from happening? And if it happens again, how can I fix it without losing anything?

If you know the approximate times when the different sites were opened you may be able to find them easily in the Library: History > Show all History.

Unfortunately, I think there isn't a definite answer because of the variables involved like the OS, security software, filesystem corruption, add-ons, extensions etc. which can all influence/alter the functioning of Firefox. You can try to set Show my windows and tabs from last time in Tools (Alt + T) > Options > General > When Firefox starts:. You can also try to find a suitable extension that may provide a second line to fall back.

Unfortunately, I don't think there's an answer. I simply do not like it when my computer does things by itself without providing an "undo last command" solution.

Please also try this, if you haven't set new tabs: Help (Alt + S) > Troubleshooting Information > Open Containing Folder (this opens your Firefox Profile Folder) and exit Firefox. Copy the sessionstore.bak and sessionstore.js files in the opened folder to another location and right-click each of the files in turn > Restore previous versions. Look for a time corresponding to the time when the lost tabs were open and Restore those. If unsure, you can try different time points. Please note that restore may not be present on every Windows 7 OS versions.

Firefox Profile Files