Hmmm. Hard to get over that dislike of an interface. I have found one plugin killed Firefox performance for me but I’m generally happy with the layout so haven’t messed with it.
On how important it is: I used Chromium for a while but went back to Firefox because I read someone somewhere say “If you don’t use Firefox, there eventually wont be a Firefox to use” and that was enough for me to switch back.
If you’re donating to Mozilla though, I guess they’re more than happy with that even if you’re not using the product.