I've used Firefox since the very early days (Phoenix 0.3) and I can honestly say it could never be described as a "fast browser" - before Firefox I was using Opera which was much faster and much more lightweight. If anything, I would say Firefox is faster now than it used to be, but perhaps now the difference is it's easier to make an unfavourable comparison to the WebKit browsers which didn't exist back then.

The audio/video tag situation is messy, but I hardly think solely blaming Mozilla for it is fair. Apple won't support Ogg because of submarine patent concerns (although I doubt them already having a license and having millions of devices with H.264 hardware support hurt) and Mozilla won't support H.264 due to known patent concerns. It's hardly the overdramatic "abuse of market position" that you make it out to be, especially given that their intentions are also somewhat altruistic.

I also don't see what a Firefox fork would achieve. I doubt anyone is going to magically be able to make Firefox faster if Mozilla's full-time staff is unable to, and I doubt ignoring patents or doing messy hacks to integrate OS codec support is going to win widespread favour given the current position of the video/audio tag.