Okay, themes don't all suck
<Diamond Joe Quimby voice>
Ever since I claimed all themes suck, I have taken a lot of heat. So, I am flip-flopping!
</Diamond Joe Quimby voice>
Orbit is very nice. The buttons look like candy.
Update: The Orbit homepage has a newer version available.
I would like to skin Adobe Acrobat
Orbit is really nice - my theme of choice since the release of Moz 1.0.0. Modern was getting boring ;)
Sinner that I am, I use the IE theme ;)
I also like Orbit very much. I could use a "Decrypt" button for mailnews as it is used in enigmail though :).
Orbit is fantastic. Lopburi Flat is nice and minimalistic. IE is just funny.
NavZilla, http://www.zukes.com/mozilla/navzilla/, is great for Mac OS X users. It gets a lot of Chimera's look (Chimera being the Mac OS X native browser using Gecko that's under development), and is usable today. With Mozilla 1.1alpha or the hack 'Silk', text in Mac OS X Mozilla is as sharp as Chimera or OmniWeb, making a pretty good overall OS X experience.
The NavZilla theme has me at just-about-full-Mozilla-convert now.
Orbit * has has a decrypt button since [at least] the version(s) of 20020627