I would pay for a browser that was 100% ad- and tracking-free. I pay for an email account. I pay for YouTube. I pay for several streaming media services. I get that people are used to browsers being free, but no reason that can't change.
Apple tried this with iOS 2 and 3. Minor versions cost users roughly 5-10 USD per update.
Therefore many users did not install the latest OS on their devices. The cost, although small, was a barrier for many people.
Apple quickly pivoted and now all software updates are free of charge to all supported devices.
If Mozilla starts charging for Firefox, I predict either people stick with the oldest version that is free, or stop using Firefox and use a fork that maintains its free (in cost) license. Or maybe only 2% of users convert to a paid version of Firefox.
They stopped doing it after they lobbied congress to change the law.
If firefox asked me once a month "Enjoying the entire internet? Is it worth $1 to you?" I'd press the button often.
Apple charging for updates is idiotic because as a user I don’t have a choice to go use a different OS.
Adding security updates and new features costs time and money.
You don’t even have to touch their services revenue to consider ongoing iOS development a significantly successful return on their investment.