Another huge change to the look and functionality of Firefox 3 is the new and improved Location bar. Not only can you find a web page in your history by address, but you can also search the location bar by page title. It's smart, too, so the more you go to a site, the higher it'll show up in your results. - Mitchell Tsai via Bookmarklet
The biggest change in Firefox 3's Options window comes through re-thinking the Feeds tab as a complete application-management tab. You can now easily set how you want Firefox to handle any filetype through this interface as opposed to just tweaking how it handles feeds. - Mitchell Tsai
Add-ons dialog hit the weight room, beefed up, and added a Plug-ins tab and the Get Add-ons tab, an add-ons store of sorts. Instead of searching the depths of the internet when you're looking for a new extension, you can go straight to the Get Add-ons tab, search for it, and install it without ever opening a web page. - Mitchell Tsai
Completely revamped the look of the bookmarks manager and included support for some of Firefox 3's newer bookmark features, like smart folders and tags. (As you can see, I haven't tagged many bookmarks yet.) - Mitchell Tsai
You can easily add or edit any bookmark while you're visiting a bookmarked page by clicking the new star in the address bar. Clicking it automatically bookmarks an unbookmarked page or provides you a simple avenue for removing a bookmark or adding tags. - Mitchell Tsai
Downloads manager has added a slight visual refresh, swapped out text links (like Cancel) for some buttons, and added a handy little search box to the bottom of the Downloads window to filter through your recent downloads. - Mitchell Tsai
Password Management: Firefox 3 provides a much less intrusive mechanism for asking you what you'd like to do with the login credentials you use to log into a site. Rather than triggering a pop-up window that requires a response before you can continue, the new password bar allows you to continue on your merry way without attending to it immediately. You can even ignore the prompt altogether if you prefer. - Mitchell Tsai