One major improvement which my wife mentioned before as the reason she preferred IE7, there's a "new tab" quick button.

Download link: www.getfirefox.com

Interesting, Microsoft didn't bother making a compatible version of their silverlight control for Firefox 3.5.

I just installed and it seems faster (but truly, I have no real measurement - I thought it was adequately fast before).

I quickly added two new and cool add-ins:
Personas for Firefox - easy themes
FoxTab - cool animated tab management

I also have running UPromise's add-in, and Ad-Block-Plus.

Cheers!