I use FF4 Portable with Firebug and Flash (not that I like Flash
Flash doesn't like proxy servers &/or remote instances of XP as exemplified by our Library system terminals.

Other than finding that FF3.XX.XX portable wouldn't run on XP that that ddn't have internet connectivity I find them good. FF4 has cosmetic differences but otherwise it doesn't feel different with the surfing I do, maybe faster to load, but the Corsair Flash Voyager stick I run it on has a limit of 30Mb /sec which is the fastest USB2 can go. So maybe I wouldn't see much improvement.

The thing about the portable version is that I take all my history and passwords with me. And with a custom homepage on the stick I can have many favourites set out in 2D.