Would any of these soft forks survive without Mozilla working on Firefox?

Depends, will I win the jackpot?

The forks do not currently have the manpower to take up the full maintenance of a browser but that does not mean it's impossible that they'll be able to rally enough developers in case Mozilla implodes. A lot of people want a truly free browser to exist. Currently Firefox (barely) manages to fulfill that role and keeps many of those people from spending their time/money on alternatives.

No