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.