Interesting, I've personally had the exact opposite experience: Chromium hardware decoding was always half-broken for me and nearly impossible to get working, whereas in Firefox, I had to modify a single setting in "about:config" 5 years ago and it's worked perfectly ever since.
I guess my caveat always is that I use Fedora (which does not include hw codecs by default due to licensing) and have to manage mesa/ffmpeg stuff myself. Adds a bit to the frustration (though it's self-inflicted so I don't usually complain :)
I'm on Fedora too, so I probably should have said "getting hardware decoding to work in Firefox is no harder than getting it to work with any other Fedora program" rather than claiming that it's actually easy :)