As I write this, I'm on the third beta of Firefox 3. And I must say, so far it seems awesome.
Improvements are legion. First off, it would appear that I was mistaken earlier and it was either not difficult for Mozilla to implement native Linux theme support, or they deemed the investment worthwhile. I'm currently looking at my default KDE icons straight in the face, so I guess they met that goal on all three platforms. Kudos.
Firefox feels faster. I don't know how much of this is the placebo effect but the whole pages seem to be loading quicker. I've yet to really stress-test beta 3 -- as in, have 20 tabs open for an hour or two -- but early reports are that Firefox 3 doesn't leak memory as much, and so shouldn't slow down over time to the extent its predecessor did.
Probably my favorite improvement though, is that now when my mouse hovers over the latest xkcd, Firefox no longer cuts off the long strings of title text that accompany the comic.