I think the last developer who cared was Firefox and even they gave up.

Firefox has native vp8/webm support. I know as I use it when I visit Youtube pretty much everyday and watch webm (vp8) videos. I don't have flash installed.

I don't think that's what he meant. Firefox was the last browser to refuse to use (not just ship, but to even use already-installed) patent-encumbered video codecs. i.e. even if you had an h264 decoder on your PC, Firefox would refuse to play h264-encoded videos. This was their (very obstinate) stance for years, and has been a source of much consternation within the FF community, as theirs was the only browser to refuse to play many online videos not available in vp8 or theora. They finally gave in last month and will now use an h264 decoder if such a decoder is installed on your PC (though FF does not ship with, nor (to the best of my knowledge) will ever ship with, such a decoder).

Does YT default to this if you go there with a newer Firefox?

No, but you can opt-in at https://youtube.com/html5

I would assume so, since when I clicked at the link sp332 provided, it says that I'm not part of the HTML5 trial, and asks me to join.

Well Google promised to support them then they backed off.

They did support them, they released VP8 royalty free and supports it on Youtube. What 'support' were you referring too?

H.264 has never mattered for developers either. Windows, OSX, iOS and Linux have all had OS libraries that developers could use to handle H.264 content. This will continue with H.265.

It's mattered for professional developers. It was an issue at my last^2 company, which was not exactly large (~100 employees).