(Happy Waterfox user; my "one feature" was multirow tabs, and the general betrayal of the Quantum rewrite. "We will add these features back as extensions" my unsupported ass. Bring back UI modding. PS: Same goes for Firefox Android, which I've thankfully found an old APK for.)
Not sure how much this will help you, I don't know which features you're keeping the older version of Firefox Android around for. But have you looked into Iceraven browser?
This repo (and that subreddit) has all the info on setting stuff up in Firefox: https://github.com/aris-t2/customcssforfx and even has a CSS example for doing multi-tab lines: https://github.com/Aris-t2/CustomCSSforFx/blob/master/curren...
(Also Firefox breaks it once a year or so.)
Not sure how much this is lack of developer effort vs lack of support from Firefox.