With Firefox at 2.93% [1] percent market share, doesn't seem like many people would put it in the MVP.

1: https://en.wikipedia.org/wiki/Usage_share_of_web_browsers

What is the market share among people who buy and use password managers?

I also don't get why pay for a password manager when there is a free one from a trustworthy entity, Mozilla Firefox, that works on android, windows, linux, iOS and MacOS.

The answer is: features. People who pay for a consumer product usually get some added value with it that you cannot find in freeware.

But what features are unique to the payed ones? The Firefox one already checks if your accounts was found on a leak, it has the complete set of feature you would expect to "CRUD" your passwords, integrates with the virtual keyboards in iOS and Android so it's easy to fill login forms on any app and by virtue of being part of the browser has perfect integration with the browser, autogenerates random passwords for you and finally it syncs all your passwords across all your devices (plus all the other things that firefox sync syncs). And again, works on all relevant platforms no exceptions.