Where they really need to improve performance is on mobile. On my old phone, I never really noticed how bad it was aside from the high energy usage reported by Android for Firefox. I recently got a new phone tho, which has a 120hz screen and yesterday I tried Cromite, a Chromium fork for Android, which improves privacy and adds adblock. I tested it against Firefox (Mull to be precise), which is what I use as my default browser, and I noticed that scrolling pages was way smoother on Cromite. First I thought that maybe Firefox is just running on 60hz but scrolling through settings and the like was perfectly smooth, just webpages felt laggy. This means that Firefox is simply very slow at rendering pages on mobile, to the point that it can’t keep up with my screens refresh rate at all.
Firefox on desktop is great and I really wanna use it on mobile too but I’m honestly contemplating just switching to Cromite. The only feature I’m missing in Cromite, that I can get for Firefox with the extension Libredirect, is redirecting to privacy respecting and lightweight frontends / proxies, like Reddit to Libreddit but it might be possible to add a userscript to Cromite with that functionality.