None of these projects have enough resources to keep up with web standards, and patch security bugs on their own. So forks are either almost the same as Firefox, or may be less secure and/or doesn't work for as many websites and addons as Firefox.

Waterfox is owned by an ad company, which IMO is enough to disqualify it. Sure it's probably fine now, but it'll assuredly have problems soon.

Of the four, I've only ever heard of SeaMonkey before. I'll definitely check them out though.