IIRC, there were other reasons not to support using the OS codecs. H264 license fees are not an issue when you just use the codecs the OS provides, as they are already covered by the OS or codec vendor. With all the Google millions that Firefox gets each year, the license fee should not really be a problem anyway.
What I understand from it, is that it would take a lot of effort to use system codecs for different video formats, while still providing the complete set of HTML5 video functions for every format.
I never really bought this argument since IE, Safari and Chrome appear to handle this just fine, if you have Perian installed for example, you can even play HTML5 WebM directly from Safari. So I think we can safely conclude there were no other arguments for Mozilla's decision not to support H264 through system codecs, besides ideological ones.
Personally, I think this is great news. Everyone should just stop complaining about the supposed H264 patent issues, standardize on it, and move on. The patents have never been a problem and they never will, you only need to pay license fees if you make money off of H264, in which case you should just pay up, technology like this doesn't fall from trees. The moment WebM/VP8 would gain any traction at all on the internet, it would prove to be a patent minefield just like H264, as you simply cannot make a modern video codec without violating any patents these days.
The real issue with the H.264 patents and why it is anathema to any libre ecosystem is the impact of the H.264 patents on video creation, encoding and distribution. Here's a thorough discussion of the issues (created by someone who actually took a substantial amount of time to talk to a representative of the MPEG-LA for clarification):
To me, this is a key conclusion:
"Related to point #2, it may not be possible to release an H.264-encoded video under, for example, a Creative Commons license that allows commercial usage. More precisely, should you release your H.264-encoded content under such a license, it would not be legally usable under such a license. This cuts out a large portion of options related to how you may share your creative content."
I find the idea that we should standardize on a video codec with that sort of usage restriction completely incomprehensible. I just do not understand how people can accept a data format whose licensing doesn't just constrain people who produce encoders, decoders and other related tools and technology, but also constrains end users.