SessionSaver is a popular Firefox extension which allows you to seamlessly save existing session and restore it automatically at startup. You can also name the session and retrieve them anytime in future. Unfortunately SessionSaver isn't available in Firefox 2.0.x and is likely not to be made available. However there are two good alternatives.

Firefox 2.0.x provides basic session saving capability automatically. Go to your preferences and click on Main tab. In Startup section change "When Firefox starts" to "Show my windows and tabs from last time". That's it! However you cannot name the sessions for future retrieval. To do that and to get other options I recommend SessionManager extension.
Install the SessionManager extension and go to its preferences to setup properly. Not only can you save named sessions, you can also save list of closed tabs etc. You can also ask it to reload each pages on restore and lots more. In short it is SessionSaver revamped. I have used it and have only good things to say about it. Give it a try.