Sure, there is no official bug for this. The only way to see it is to check the dates when the new API was applied - its far too late after releasing versions already supporting brotli, but this commit was not backported anywhere so in a real life, its broken in FF. Just get an nginx, nginx brotli module from google and use FF 45. Good luck :)
https://hg.mozilla.org/releases/mozilla-aurora/log/10e1774de...