Software is not a fast business, VA-API came around in 2007!
If you had supported this early, it would've given users nothing but trouble, because of all the bugs. Then again, if nobody supports it, nobody will use it, bugs will not be found and fixed.
There is a library that maps VA-API to VDPAU (their other video acceleration library), but that doesn't support DMA-BUF either.
I wouldn't hold your breath that this would ever be supported. It'll probably be easier to modify Firefox to use NVDEC directly, given that FFMpeg already supports that. But I don't think that'll be easy given their reliance on DMA-BUF for composition.
Edit: not so, see below