If you find that Firefox 4 no longer saves your tabs when you exit this is easily rectified:
Open Firefox and type about:config as the URL and then press enter.
Click the I’ll be careful, I promise button to continue:
Type browser.showQuitWarning in the filter until browser.showQuitWarning is the only item that you see below. Double click browser.showQuitWarning to change it from false to true:
The setting should take immediate effect.