Firefox's hopeless video and audio implementation (especially on Mac OS X) remains a deal breaker for me. It means resorting to Flash in a lot of cases, which means I use another browser.
I'm glad to see strides being made in JavaScript performance, but basic browser functionality is far more important.
HTML5 video and audio in Firefox works perfectly in every site I visit. What problems do you hit? I haven't used it on Mac so perhaps the support is less good there.
Some sites don't attempt to use <video>/<audio> in Firefox for me, but that's a site issue, not a browser issue - h264 and webm both work fine.
I'm glad to see strides being made in JavaScript performance, but basic browser functionality is far more important.