I installed it straight over the top of FF2, it clicked and buzzed and whirred a bit...then updated my plug-ins (after asking nicely, of course!) and now apart from an extra tab on the quick-links bar ("Most Visited"), and the faster speed...no differences noted yet..even Java didn't need updating! (Not been on any "Flash" sites yet tho...)
Spell checker still works...my preferred skin still shows correctly, and all other extensions and plug-ins are enabled fine...well, except for the WGA Plug-In, but don't want/need that anyway!