Firefox 3.0 official release is today but it seems that the Mozilla site is having problems since i cannot download any add-ons for FF 3.0. :( I guess the world record for most downloads in a day is gonna have some problems.
If you want the new version you can try to download from here
I have found that while IE is my preferred browser that some of the development tools that Firefox provides via add-ons have become invaluable to me during my day to day development tasks. Things like Firbug (with the real-time editing) are indispensable when trying to figure out what SharePoint CSS I need to modify to get a specific look. When the site comes back up I’ll post the add-ons I use in my day to day development.
FYI – Internet Explorer 8 is going to have some of more popular add-ons in their build as well. Last time I had checked, they had a developer (firbug’esk) feature that allows you to do in place editing to see what a page looks like as you make modifications to it.