I had one major disappointment when I installed Firefox 4 after using all the previous versions of Mozilla – it did not offer to save sessions and open tabs anymore. I read several solutions online for it and most of them presumed that my Firefox profile and preference data were corrupt and since I had still some other Firefox versions on, I believed it. The suggested solution (which appeared to be incorrect) was completely re-installing Firefox. Which I did.. but, naturally, it did not help.
The correct solution was very simple (found here):
1. Enter “about:config” in your Firefox 4 address bar, hit Enter and accept the warning
2. Find setting called “browser.showQuitWarning” which by default is set to “false” and change it to “true”
That’s it! Now Firefox 4 will ask you if you want to save your session when you are about to quit.
P.S. Use at your own risk – if you have this issue, causes of it may still be different than in my case.