Will anyone pay developers for FireFox extensions?

I would not. I might, however, donate.

Paying is upfront, before I even know if the add-on will be useful. I don't like paying for anything upfront without some kind of evaluation.

Also, seeing as my use of the add-on depends on the continued existence of Firefox as well as the developers commitment to keeping it up-to-date with breaking changes in Firefox, I don't see a long life expectancy for add-ons anyway. I've already had some favorite add-ons fall by the wayside. I have no interest in paying for something that suddenly becomes unsupported without explanation 6 months later.

Finally, my use of add-ons is relatively limited anyway, and most of the ones I do use have very obnoxious features that I wish I could do something about. Right now, the only things I can see giving any money in any way for are Firebug, its extensions for analyzing page speed from Google and Yahoo (though I'm not as likely to "donate" money to a major corporation), and Web Developer.