Firefox 2 was released last year, but since I was in the digital wilderness at the time, I sort of missed it, and only got around to upgrading recently.
One thing I don't like about the new Firefox is that the close 'x' icon is now on each browser tab. IIRC early Mozilla browsers did this and I think it's a pity they've regressed. I like having the single close button to the far right, because it's easier to close a number of tabs at once, without having to move the mouse. I find it a bit frustrating when I want to close a lot of tabs and have to move the mouse across the screen to catch 'em all. (Update: there's an about:config setting to restore the behaviour, and more, see comments for a link. Thanks Bryn!)
What is quite nifty is that if the browser crashes (or if it's open when you log off), it prompts you to recover your session, and will re-open all the pages you were visiting when the browser closed/crashed. That feature can also be quite useful for normal browsing, for example, when you're really busy with something and then need to reboot because that's what you sometimes still have to do with Microsoft Windows.
Of course, you can save everything with the Bookmarks > Bookmark All Tabs... option, but if you think restoring your session is really cool and worth using every time you restart Firefox, you can specify that too. Perhaps this option's been around for a while, but I never noticed because Firefox never prompted you on a restart? Either way... it's nice to know the feature is there. Just go to Tools > Options and in the main tab, from the 'When Firefox starts' drop-down, choose 'Show my windows and tabs from last time'.
What was that about never posting tips anymore?