I've been using Firefox 3.0b2 for a couple of days, and it's really nice. Most of the important extensions have a compatible version around somewhere for testing: Greasemonkey, Firebug, Stylish, Adblock Plus, DownThemAll, Secure Login.
One of the most interesting changes in Firefox 3 is that pages reflow differently when you zoom in or out: instead of just the text resizing, the whole page zooms. It means less things break when designers use pixel-based positioning rather than ems (but they still need to account for people who have set a minimum font size).
I also took the opportunity to write a nice, simple del.icio.us posting extension (derived from del.icio.us post, but simplified).
There's one bug in Firefox 3 that needs to be fixed for it to work well there, but [update: fixed now] it works fine in Firefox 2 (except for the lack of notification window on OS X). Please note: this is the first time I've produced a Firefox extension, so it doesn't have auto-update or anything like that yet and you have to manually add the button to the toolbar - this is an alpha release.
Update: newer versions of this extension are available in the Firefox Add-ons sandbox. Please rate and review the extension there so that it can be moved out of the sandbox.