I didn't need to tweak any config switches.

I think you might be overtuning your browser. Probably best to reset to defaults and go from there.

I doubt that I actively disabled webm. But yeah, it's been years of switching this or that, firefox adding or removing this or that, etc. That does add up. One of these pages seems to imply that "webm" was turned off when it first came out in firefox. And until not so long ago, Google didn't send everything in webm.