Not bad. There are, however, a couple of bugs that really need to be ironed out. I'm running Firefox 3.0.5 on Ubuntu 8.04, and I've noticed a couple of funny little quirks:
1) Whenever the back button goes from grayed-out to green, that toolbar (the one with the buttons, the URL, etc.) vertically widens just a little, and it's supposed to stay at the same height throughout.
2) Whenever I hover my mouse over the menubar items (File, Edit, View, etc.) or the menus that appear under them, the main menubar labels completely disappear. These need to be fixed, too.
Also, as a matter of personal preference, I would prefer to see the forward button be green, just like the back button, and also to see the original yellow "home" icon, too. But that's just me and my preferences. Other than that, I like it. Just those little bugaboos that need to be corrected, and I think you'll have yourself a first-class theme. :-)