Many people would pay for firefox so long as it supports their pet features and they all have their own pet features. And it would still be less than they'd get from advertising.
Invariably this is true, but I think had a payment model been adopted when Firefox itself was gaining traction, they wouldn't need to stray from that winning product model - privacy focused, user control, advertisement free, web standard honoring.
As it stands now, they are obviously wanting to monetise their product but for whatever reason don't want to open up to the option of a retail/consumer grade price. Probably it is because they'll make more from feeding ad companies.
I think back when firefox was gaining popularity there wasn't really a public awareness for the need for a privacy focused browser. In other words, even fewer people than right now would've been interested in buying it because they didn't really think they needed it.
Once someone gives away a roughly equivalent good enough product for free charging for a competitor becomes incredibly difficult and that ship sailed when MS bundled IE with windows.
As it stands now, they are obviously wanting to monetise their product but for whatever reason don't want to open up to the option of a retail/consumer grade price. Probably it is because they'll make more from feeding ad companies.