Some possibilities:
Does it still happen if you run Firefox in its Safe Mode?
It could be a bug.
Clear your cache. Firefox> Preferences> Advanced> Network, Clear Now button.
This can be a problem with saving the file sessionstore.js in the Profile Folder
Delete the file sessionstore.js and any existing files sessionstore-##.js with a number in the left part of the name like sessionstore-1.js .
Does
this thread help?
I got this stuff from the Firefox forums. It might be worth a visit there if the problem persists. They allow guest/anonymous posting on their Support forum, so you don't need to register to ask a question if you don't want to.