I noticed upon upgrading to Firefox 5.0 that it no longer has the option to “save your tabs” / “save session” when you are closing.
Save Session Tabs in Firefox 5.0
The way they do this is a little different. Follow the instructions below.
With your tabs still open, simply close Firefox 5.0.
When you re-launch Firefox, you have two options. If you left Firefox with the default Home Page on, then you can simply click on “Restore Previous Session.” If you changed the default home page then follow step 3.
After you re-launch Firefox, go up to the Firefox tab, go to History, and then click on “Restore Previous Session.”