I just got round to updating to Firefox and the latest Thunderbird (there is a splendid tool called Mozilla Backup around, which lets you backup your profiles properly, just in case anyone lacks it). I've not noticed many differences from Firebird, but then have been in London for the last few days. One thing is the icon is way better.
My bro uses Mozilla still, and every time he visits complains that I close my browser after each time I use it. He then sees how fast Firebird/fox loads on my machine (partly it is much faster than Mozilla to load, and partly my computer is actually a lot better than his) and he kind of shuts up then.
Anyway, I couldn't imagine not using the Mozilla dev project browser/email client, whatever the fuck they call it at the time.
As for skins, I am very utilitarian, and nuke all but - back, forward, reload, top and gome, and make them small icons with no text. I then stick the address section on the same line, so the menu and the address/buttons/google box are all on one line. This just is least cluttered and offers most space for the page I find.