Just had a look at the first alpha from Firefox 3, codenamed Grand Paradiso. Yeah, Firefox THREE!
The Mac version has something that other platforms won't feature: a switch in the way browser elements (also known as widgets) are drawn. For Firefox 3, they will use the Cocoa widgets, which were used previously in the Camino project (another branch of Firefox aimed at having a real Mac OS X look and feel; I talked about Camino before).
Although the notes point at Cocoa widgets being slower than Carbon ones, I felt the performance being quite better in my tests. I tried with some pages that render slowly in Firefox 2, like Google Analytics or Google Mail. In Grand Paradiso, they are smooth, even with some nice deceleration when scrolling up or down.
I don't know how much of that performance increase has to be attributed to the new graphics rendering engine, Cairo. Although there are some rendering glitches, like some underlines which have 2 pixels instead of just 1, it looks better in general. For example, xplsv.tv, which has a small typeface (verdana 10px), now it's way more legible, when in Firefox 2 it looks a bit too crowded and excessively bold. Space between lines has been increased as well, so maybe people will have to reconsider their css if this is to be maintained in the final (and they are picky about it; in my opinion it looks better like that).
Unfortunately it still has those rendering bugs and none of the extensions were compatible. I could fix them to be compatible as I did with Firefox 2, but I'm not feeling like hacking today, so I'll stay with Firefox 2 until they release something a bit more stable. But it looks very very promising!