[mcpherrinm]

It would be possible for Firefox to do use the system video codecs. This is the approach Microsoft has chosen to go with for IE9. Mozilla has chosen not to do this partly to prevent h.264 in Firefox from catching on, but also because doing so would surely lead to to the same problem as before: Requiring end users to install things to play video. We're back in the codec nightmares we had that flash replaced when people stopped using <object> and <embed> for video on web pages.

[ZeroGravitas]

Microsoft hasn't chosen this approach in IE9.

They originally were going to only use H.264, and only provide IE9 on operating systems where they themselves had provided the H.264 codec (Vista & 7).

They've since committed to supporting a user install of WebM but I've seen no technical details of how this will pan out e.g. what if you install more than one WebM codec from different sources? They have been clear that no other codecs (e.g. DivX, WMV, Dirac, Theora) will be picked up regardless of built in support or user installation.

As you rightly note Mozilla, particularly on XP which makes up about 60% of their user base, would be relying on god knows what kinds of codec packs that users have acquired over time, a known malware vector and so bundle their own codecs.

[recoiledsnake]

How can Firefox using the "known malware vectors" make things any worse if they're already installed on the user's system?

[ZeroGravitas]

The "vectors" aren't installed, vectors are the way that they get installed. Training people to install codec packs when prompted in order to see a video is considered risky, since the next time they see a similar message it's likely to be someone up to no good.

[nanairo]

My impression is that everyone will have support for h.264 on the computer anyway (it's such a common codec). And even if they don't WebM could be used as a fallback. There is no reason why we could not have support for both, and Firefox will only support h.264 via the OS.

[ZeroGravitas]

Realistically Firefox will play H.264 via Flash just the same as IE 6, 7, 8 and so will play any video on the net (including those in old Flash formats) till those 3 browsers collectively drop below 5% or so. By which time the conversation will be about H.26_5_ and a royalty-free successor to VP8.

[recoiledsnake]

The problem is that Firefox will not and won't load Flash for the HTML 5 <video> tag which references a H.264 video.

[ZeroGravitas]

The solution is that IE 6, 7, & 8 won't load Flash for the HTML5 video tag either, so the same Flash fallback (probably delivering the same H.264 file given to HTML5 browsers) works for all of these highly popular browsers which currently account for something like 80-90% of all browsers between them.

Firefox only has to worry about niche, tech-forward sites that feel they can disregard all pre HTML5 browsers and also actively choose to ignore Firefox (and Opera) as well by not providing a WebM fallback video. No ordinary business can afford to simply refuse to deliver a Flash video to 1/3rd of their audience when they've already built it and are serving it to another 1/3rd on older version of IE. (This obviously occurring at some future time when the HTML5 video delivery is preferred over Flash for any platform other than the Apple ones that don't have Flash, otherwise the Flash would be going out to 99% of browsers anyway).

There's not much downside for Firefox except pissing off people who really passionately hate Flash, but are quite happy with H.264's patent situation. I'm thinking the crossover is pretty small on those two populations and probably shrinking greatly now that Apple has let Adobe use their hardware acceleration API for H.264 decode, certainly not big enough to derail a browser used by tens of millions of ordinary people.