Personally I really enjoy Opera's functionality, so I decided to try and make FF3 act kinda like Opera. I guess one of the good points is that you can do that, change things up and junk.
It took about 5 'extensions' to make it feel right to me. Here's the list:
AdBlock Plus - Gives the ability to block unwanted content.
All-in-One Sidebar - Adds the quick-click sidebar functionality.
Fast Dial - Basically Opera's Speed Dial, but with more options.
FireGestures - Like Opera's gestures but again with more options.
Undo Closed Tabs Button - Not included with FF for some reason.
Looking through the popular 'extensions' I found these ones:
DownloadThemAll! - Opera almost has this functionality, but this is so much easier.
NoScript - Turns off scripts for all sites and then you allow sites you trust.
PicLens - Kinda neat but gimmicky GUI thing for looking at pictures on the web.
Yet Another Smooth Scrolling - Gives options for smooth scrolling.
All in all, I'm both impressed and unimpressed. Part of me doesn't like it because w/o extensions the browser is just barren and empty. Opera is a full featured web browser, email client, IRC client, and Torrent manager. I admit it's sometimes hard/impossible to add things to Opera, but it's probably better for security and privacy anyways.
So FF has functionality, but Opera has performance & security. I'm kinda torn.