Author of that PR here. It may help Firefox some, but in my experiments I'm still seeing that driving canvas animations with Elm performs much better in Chrome than in Firefox.
If there are any Firefox engineers reading this, maybe there is a good benchmark to be extracted out of this observation? I'd be interested in discussing this with someone. Feel free to reach out at the e-mail address in my profile.
https://github.com/elm-lang/core/pull/25
I believe the original improvements were done here: https://github.com/jwmerrill/elm-animation-frame
I modified a game I wrote to use the modified run time. I can see a huge difference between the two in Firefox.
Unmodified: http://jcollard.github.io/elm-flyer-old/
Modified: http://jcollard.github.io/elm-flyer/