You can configure the Firefox web browser to reload the last session on start up. That’s a custom setting that you need to configure under Firefox > Options, the General tab there and the selection of Show my windows and tabs from last time. When you do that, the browser opens all tabs that you had open when you quit the browser. On my computer, it seems to load the last open window more often than not, which can be attributed to the fact that you can only close one window at a time. If you’d kill the Firefox process, or have the browser restarted automatically, you’d probably end up with all windows being opened again as well.
It has happened in the past on my system, that Firefox would not load the session. The browser instead displayed the selected homepage instead with no sign of the tabs that were open in the last session
Firefox comes with options to restore the windows and tabs of a previous browsing session. You can access the listing under Firefox > History > Recently Closed Windows. Here you find the windows listed by the tab that was last active. When you click on one of the sessions, the whole window with all of its tabs gets restored.
Remember that those information are only displayed if you do not clear your browsing history on exit. If you have configured Firefox to do that, you won’t be able to restore sessions. This is handled by the browsing history setting. You can configure everything else to be deleted, and still be able to restore sessions in the browser. If you select to delete the browsing history, you cannot restore sessions. You will in fact notice right away that you are starting with a single page in the browser after you have enabled it.
You can however disable remember by browsing and download history on the main privacy options page without losing the functionality to restore sessions in the browser. Feels kinda strange though if you ask me.
If you have very important sessions that you cannot afford to lose no matter what, you may want to consider upping the ante by installing an extension like Session Manager which saves sessions independently. Handy to avoid total disaster if the built-in session restore is not working correctly, or if you want the browsing history to be deleted on exit.via[ghacks]