I don’t know of any features that would improve Firefox’s market share to any significant effect. Tbh I haven’t even noticed any feature removals, but that’s just my day to day use cases: I bookmark things, I have multiple windows with tabs open, I install privacy add one and ad blockers, I sometimes open the console and mess around with websites. I don’t think the vast majority of the market even does a portion of what I do, much less any more obscure features. So I’m not inclined to believe that any removals changes Firefox’s market share to any significant effect. They just have no realistic way to monetize a browser. I doubt safari or chrome are money makers either, but they have anticompetitive conglomerates using those browsers as customer lock in and not a core product. Completely different needs there.
No single removal changed Firefox's market share to any significant effect, but it's death of a thousand cuts. Eventually, they're going to have to start doing things again that gain them users rather than lose them.
(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.)
You can still mod Firefox's UI pretty easily, its all defined by CSS files and not too difficult to customize. I for instance actually did the opposite of what you wanted and completely got rid of the tab bar cause I use Sidebery for tab management: https://i.imgur.com/cHz1clI.png Theres a whole subreddit for it even: https://www.reddit.com/r/FirefoxCSS/
>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?