I'm curious what you find "hideous". 3.6 -> 4 was a pretty big overhaul, but I think it looks pretty good. I customize a bit (small buttons, get rid of the search box & home button) and I may be baised.

I'll say that Firefox isn't perfect and there are quirks & oddities with bits of the interface. I'm sure there are bugs for most of them though.

(note: I work at Mozilla on Firefox, so while I'd normally be curious, I'm invested in fixing problems!)