I'm comparing to Firefox 3.6; Firefox 4.0 isn't released yet. But everything I've read indicates that Firefox 4.0 will still have a single-process, non-low-integrity process model, with all the security and reliability implications that go with it.
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...
Understandable, but still, when it comes to comparing how IE 9 stacks up to "Firefox" on rendering engines and memory usage, it doesn't seem particularly meaningful to compare the brand new IE 9 to the old Firefox just because IE 9 released a single week before the next major revision of Firefox is slated to. Not that you were writing a New York Times article here or anything. :)
I certainly understand your other point about the single-process design.
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...