Yeah, but since it doesn't have an entire dev team committed to it, the fork dies and/or you're using an out of date browser with all sorts of security vulnerabilities and incompatibilities. Some attempts with actual resources behind them:
Cyberfox - dead
PCXFirefox - latest merge was FF 53. Using outdated plugins (e.g. Sumatra PDF from 2014!)
Waterfox - dead
Pale Moon - Still alive. Removes a host of features (e.g. Accessibility and Parental Controls) in the name of performance, yet performs horribly in benchmarks. Slow to adopt new features.
Besides, who's going to use these? A tiny fraction of the population consisting of people reading this thread and threads like it? Who cares?
So yeah, you _could_ "just" fork FF, but it won't get you anywhere. And why the hell am I doing this? I have much better things to do with my time than maintain my own damn web browser. I am constantly amazed at comments like this which are so obtuse as to be comical.