Call me boring if you like, but I have Aronnax's GrApple Safari lookalike theme installed. It's simple, out of the way, not too flashy, and fits in with the rest of OS X fairly well.

As for extensions, I have the following installed:
• Adblock 0.7.5.4
- Should be obvious what this does
• Fission 0.9.7.1
- Moves the progress bar into the URL field, Safari-style
• Stylish 0.5.7
- Wonderful, wonderful, wonderful plugin that allows you to modify the looks
and even some functionality of any site you please via CSS stylesheets.
Examples include theming sites (Blue deviantART!) and modifying YouTube
to have a much larger video player.
• URL Link 2.02.2
- Very handy plugin that allows you to create new tabs and windows from those pesky unlinked URLs.
I tend to stay away from plugins that add functions that aren't really centric to browsing, since it moves Firefox towards a more monolithic "everything" program (which I heavily dislike). I'd much rather use a separate torrent app, music player, etc, etc, etc...