This is my setup. Believe it or not, this is actually PM. Theme is Kempelton Reloaded, which is gorgeous as is on any platform. However I installed an older version of Classic Toolbar Buttons and configured it to match the FF 3 Strata scheme. I also have some tweaked favicons too, userscript based. Since Kemp also supports Lightweight themes via workaround, I applied that color scheme as well. Little tweaking and, surprisingly the tabs bar styling looks native with it. Doesn't look quite right when some elements are moved around, but I have a few more similar Personas in that case. Few icon resources in theme itself, friend and I tweaked. Explains the nice blue close buttons on tabs, or the throbber. As far as the Firefox branding itself.. That was just a parody to see what it would look like, or what people would think. Used the resources from 3.0, before the 3.5 logo change. Compiled it with an addon called Rebrand. Tbh seeing it kinda spikes good memories to me. Like when Mozilla products didn't suck..
But yeah, that's my FF 3 clone. Looks very native with the rest of my un-eveolving, 2008 inspired desktop. Gotta love it!
@Night Wing
Admire your config. Love Mint. I remember before Australis, FF used to blend well with that desktop too, adapting GTK icon style.