For Firefox forks, it’s viable since the forks aren’t doing all that much in the grand scheme of things. That isn’t to say what they’re doing is in any way bad, it’s just that there’s no need to reinvent the wheel.

Firefox is a secure browser and already has 99% of the work done. Most changes which forks make can be done just by changing the config. Some unfortunately have to be made seperately, and that does require extensive testing. Some can even be lifted from other open-source projects.

Separating from source just isn’t viable. Something nuclear would need to happen for any fork to decide to seperate from Firefox. If we just look at the Chromium side of things, Microsoft found it easier to switch to Chromium than to keep making IE/Edge from scratch, and Microsoft surely has a lot of resources to burn.