Sorry , you probably can not get them back.

You may have bookmarked them, or have them as part of the History previously stored and still searchable, but unfortunately otherwise you are unlikely to get them back. Firefox does to a certain extent back up bookmarks automatically but not the history.

If you get problems with crashes see firefox crashes

with a bit of luck you might have a backup of sessionsstore file (either with .bak-ending or sessionstore-1/2/3...) in your profile folder. look at the changedate of these files - when one of them was last changed shortly before the crash you might restore the session. first make a copy of this file to another secure location like your desktop. then, while no instance of firefox is running delete or rename the actual sessionstore.js file and rename the backup file that you've got into sessionstore.js in order to replace it & launch firefox again