Rated 5 out of 5 stars
This has been a standard add-on for me for years. The "undo closed tab" button you can add to the toolbar will alone save you lots of menu digging time.
Session Manager lets me browse without fearing that I'll loose things. The option to selectively not reload certain tabs when Firefox restarts also keeps things a bit more sane if you've opened far to many tabs or you just don't need tabs you'd opened the session before. The session history and ability to bookmark/save whole sessions is also a great way to keep your work/life organized.
Give it a try. You'll be glad you did.