Out of curiosity: by whom? I don't think this is likely at this point, short of some truly benevolent benefactor pushing tens of millions of dollars per-year into a competing foundation.
None of the "alternative" browser companies today built upon Gecko or forked Firefox itself, because _it's really hard to maintain a browser engine_. Mozilla itself barely manages to keep up, and Apple sort-of-does (and can riff off work on Blink due to WebKit's shared history). This is also why Chrome & Chromium tend to drive de-facto standards (some good, some not so good) with an implementation-first approach.
Building browsers is hard. Really hard, especially as we consume more media and build richer applications on top of the web. Dedicated teams for video & audio, WebRTC, Wasm, CSS, JS itself (huge, let alone the security challenges), etc.