For those of you wondering what improved on Firefox 2, you can check out this link for complete details. They mentioned Improved Tabbed Browsing, Spell Checking, Search Suggestions, Session Restore, Web Feeds (RSS), Live Titles, Integrated Search, Live Bookmarks, Pop-up Blocker, Streamlined Interface, Accessibility, Phishing Protection, Open Source, More Secure, Automated Update, Protection from Spyware, Clear Private Data, An Add-on for Everyone, Add-ons Manager for Extensions and Themes, and Search Engine Manager.
So btw, I was wrong on my last post where I mentioned Tab Mix Plus does not work with FF2; it doesn't indeed, but all the functionality is now part of the core distribution, including session management.