Mozilla just spent two years telling everyone that they were not going to do this, and posting justifications as for why not. To me this is a serious blow to their credibility.
They fought 2 years for getting patent-free video recognized, probably at the cost of some of their userbase, and a lot of anger from web-developers who saw their lives complicated.
Today, they seem to be reconsidering their position because they think it's a lost fight.
How is this a blow to their credibility? What should they have done? Instantly bend over next time? Stubbornly refuse to reconsider and lose the best hope for a truly free phone OS because nobody would want to use it?
If you lose a war, it's never pretty. Kudos to Mozilla for taking a hard stance against patented standards for as long as they could. They deserve credit for that.
Because he's not saying "we tried, but it didn't work out in the end". He's saying there is no justification not to use available platform decoders. Yet they were not shy to provide them in the past. He's not addressing why those are no longer relevant arguments.
Obviously the real reason they're doing this now is because on their new mobile platform, the Gecko runtime is all there is, and without it supporting H.264 playback, the platform would be considered incomplete at launch more so than their desktop browser is for not supporting it. I get the motivation. I'm not saying it's wrong - I don't agree with it, but that's not the point here. The point is how bad it makes Mozilla look to change their stance so willy-nilly.
Yet they were not shy to provide them in the past.
The reasons are still there - it's even pointed out in the parent discussion that they're still a problem on Windows. One main difference is that Windows 7 is an order of magnitude more widespread now than it was 2 years ago - when those posts you refer to were written.
And as you already admitted - they're doing this on mobile first. There's no guarantee yet it turns out to be actually feasible on desktop if those same arguments can't be properly addressed.
But the Windows problems aren't the only justifications they published back then. In the second linked post O'Callahan writes:
"It pushes the software freedom issues from the browser (where we have leverage to possibly change the codec situation) to the platform (where there is no such leverage). You still can't have a completely free software Web client stack."
Given that it strikes me as ironic that Mozilla is now building a platform - i.e. a platform where they definitely have leverage - that is going to support H.264.
It was a stupid, misguided effort from the beginning. Due to the way patents work, there is no such thing as a patent-free video library. You can't make one. It's impossible. Any non-trivial program will have some kind of infringement and if someone's trolling for "infringements" then they will find them.
They should've worked instead to get proper buy-in from the various parties supporting the current standards to ensure that another GIF incident doesn't happen.
The difference between H.264 and "any video codec ever" that may have an infringement charge brought is that H.264 has a large, well-organized overlord in the form of MPEG-LA. They are actively looking for violations, and threats of suits from them are credible and realistic. "Any non-trivial program" may have an incidental patent infringement claim filed against its authors, but with MPEG-LA and especially a violator as big as Mozilla, you can expect a claim if you don't purchase a license ahead of time.
What I mean more specifically is that a group like MPEG-LA could be lobbied more effectively by Firefox to produce some kind of acceptable licensing model for open-source software and end-users if Firefox had stayed in the game instead of taking their ball and going home.
I think you're confusing MPEG and MPEG-LA, but either way you're mistaken.
MPEG claimed they were going to create royalty-free profiles of H.264 and then went back on that (possibly due to cunning political moves by Microsoft) and now claim to be evaluating a couple of ways forward for royalty-free codecs (a profile of H.264 again, or building on older MPEG 2 tech). It's fairly obvious that it was pressure from (primarily) Google & Mozilla that put RF codecs back on the table for them.
MPEG-LA reduced the uncertainty around their future codec licence payments, and as a result their maximum profits. Again almost certainly due to Google and Mozilla.
They didn't take their ball and go home, they negotiated and competed and as a result won valuable concessions that continue to have value as the next round of codec development begins.