I'm not sure how many there are that would pay for it, but I'm sure it's not zero.
I believe a browser is like an IDE, and I'm quite happy to pay for mine, as are plenty of other people apparently, JetBrains & Co are making good money. They are focused on their users though, which Firefox isn't.
I'm pretty sure that Firefox could get plenty of paying users at $100/yr even just by focusing on good developer experience. They don't though, and Chromium does.