Are you comparing IE 9 to Firefox 4.0, or Firefox 3.6?
Hopefully Firefox 4 will catch up to the likes of Chrome and Safari (and IE 9) in terms of JavaScript performance, judging by benchmarks on the release candidate, but I haven't seen any indication that it addresses Firefox 3's fundamental design weaknesses. Of course, feel free to correct me if I've missed something in the release notes somewhere...
I certainly understand your other point about the single-process design.