I have personally just cut over to Firefox 3 as my default browser. I was finally able to gather the core add-ons I needed to make the switch. Some of the add-ons are still in beta, but are at least compatible with Firefox 3 so most of he functionality is there.

Firebug (use the 1.1 beta) - Install Now
Firecookie (use the beta) - Install Now
Greasemonkey (supports up to FF3.0b5) - Install Now
Web Developer (current version supports FF3) - Install Now
Operator (current version supports FF3) - Install Now
PageDiff (current version supports FF3) - Install Now
Uppity (current version supports FF3) - Install Now
IE Tab (current version supports FF3) - Install Now

Not development related but add-ons I find very handy

Better Gmail 2 (current version supports FF3) - Install Now
Download Statusbar (current version supports FF3) - Install Now
Foxmarks (current version supports FF3) - Install Now
Foxyproxy (current version supports FF3) - Install Now

If anybody else has add-ons that they find useful along with information on how to get them working in the Firefox 3 please share in the comments.