These FF forks will all be nonviable within a few years of Mozilla going under. Google and Apple will keep moving the web (for better or worse), and these forks will be unable to keep up for lack of resources.
Whether they get relatively slower, or just can't support some new web tech, the writing will be on the wall.
I doubt user funding would cover more than a small fraction of the costs. Someone crunched the numbers in the Lobsters comments, and compared it to Wikipedia's funding. They concluded Mozilla would have to be more popular than Wikipedia to make it work, and it almost certainly isn't.
I think an intergovernmental EU funding initiative might work, though.
Whether they get relatively slower, or just can't support some new web tech, the writing will be on the wall.