Goddamnit, I really, really wish the Firefox UI wasn't so broken in OS X.
Deal breakers:
Middle mouse doesn't open new tabs when clicking on links
Find-as-you-type doesn't work
You can weasel around the middle mouse thing by mapping the button to splat-click if your mouse utility supports it, but then you're screwed if you want to use the X11 copy/paste buffer. Find as you type just flat out doesn't work, and that's one of my favorite features in Firefox.
I really hope the Firefox team gets this stuff straight; I love the browser and would really like to use it on my Mac. Right now I'm using Safari since it's pretty decent and middle click works, but I really miss find-as-you-type and some of the cooler Firefox extensions.