I got onto the FF91 update too, by mistake. And I am now left with this total disaster of a Proton UI too.
FF devs seem just totally tone-deaf to users not wanting Proton… If the workaround mentioned below doesn’t work, then I’ll go for the “long term support” release of FF for the time being.
I’m really pissed off about this for now. 
The intention is good but it looks very hacky and doesn’t address all the issues I have with the Proton UI abomination. For instance, it appears that the tab icons for “Playing” etc still do not show, instead having this really annoying text below the title.
There is an option to generate code for that. Third item down in the second column.