No, Firefox isn't the same thing as Twitter. But if large numbers of people show willingness to pay $2.99/mo to change the app's theme, surely there's enough privacy-conscious people that would pay similarly for a browser that was commercially incentivised to protect privacy rather than monetise it.
I would happily pay a couple bucks per month for a Firefox Pro that’s exactly the same as normal Firefox.
Provided, of course that it’s easy to start and end the subscription, and I don’t have to create a new account. Fortunately Apple provides all of this with the App Store.
A) I don't want to create yet another account and give my CC number to yet another entity who can lose it
B) I guess I don't fully trust them to use the $$ for anything that I care about. Tying the revenue more directly to Firefox IMO would send a stronger signal that this is what matters.
As far as I know it's impossible to donate towards the browser's development.