There are definitely enough users who would donate money to support a fork of a browser that pursues their interests. Not enough who would support Mozilla though.
Unlike most other software browsers are in a unique position here. They have regular users that use the software often and a lot and can do periodic donations campaigns that reach all of them. On top of that there are billions of potential users opening various niche possibilities. This may not be enough to develop a massive full blown browser engine, but definitely enough for forks.