Reference for this, please? Because I see no bugs filed on FF regarding brotli apart from https://bugzilla.mozilla.org/show_bug.cgi?id=1222541 and https://bugzilla.mozilla.org/show_bug.cgi?id=1207234 neither of which seems to match your problem description. I'm happy to get a bug filed for you so whatever problem this is can be fixed, but I need a bit more to go on here...

I did look around for known brotli+Firefox+nginx issues, but the only one that comes up is https://bugzilla.mozilla.org/show_bug.cgi?id=1215724 which was fixed in shipping Firefox months ago, so I'm assuming that's not the one you're talking about.