Chrome without acceleration ~~ Firefox without acceleration. I don't think Firefox is enabling hardware video decoding in Linux, even on a fully-free driver?

Acceleration in browser is not only about video decoding.

The composition is GPU accelerated too. Chrome has it enabled by default, Firefox doesn't.

In the end, both browsers treat Linux as a second-class citizen. To make things even more weird, ChromeOS uses the same API (VA-API) and the same driver (for Intel GPUs) for video decoding acceleration that the stock Linux does, and the video decoding is supported on ChromeOS but not on Linux... And Firefox is just plain mess.