h264 isn't going to be a practical problem for the vast majority of users, since Firefox can just use a system codec (non-Windows-users would have to make sure they have one, of course).
As for JS speed, Mozilla are very ardent in their speed claims, so it's hard not to believe they have something to back it up. It's difficult for users and external testers to figure out exactly how fast they are, despite being open source, because the Moz team is pursuing several parallel tracks to increase JS speed. There's "fat-val", "tracer JIT" and "method JIT". Each is currently significantly faster than the "normal" versions, but there hasn't been any public testing on a build that combines all three.
Mozilla claim they'll be faster than everyone else and while they may be scuppered by new advances from Google and Opera, it seems reasonable that they will at least be faster at launch than where everyone else is now. That alone would ensure "next-generation JS performance".
Where they trail Chrome is in "use speed". Chrome starts and shuts down a lot faster -- and I think that's going to be a problem for Firefox moving forward (more than it already is).