What Firefox really needs is a little icon (preferably displayed prominently in the toolbar) that appears when videos are not being decoded in hardware. I'm sure they've lost users who just think Firefox is "super laggy with YouTube" or that "Firefox really eats battery". An indication that something is wrong and the browser knows about it prevents the assumption that the browser is just buggy/slow/etc; the word-of-mouth changes from "Firefox sucks" to "Firefox told me it couldn't hardware accelerate my videos or something?" Still not positive, but potential users will hear that Firefox might not work with their hardware, rather than that Firefox is buggy, slow, and unreliable.

And I know technical users would appreciate it. I still don't know what is and isn't getting hardware video decode, or whether I actually need a particular about:config flag or system package, and I have tried hard to figure out.