Let me pay for Firefox

Yes!

Telegram is a good example of a public app that was free, and bleeding, they introduced paid features and are profitable.

Never they forced you to pay for existing stuff, nor sold your soul on the way to profitability.

Telegram also introduced ads, but yeah, it’s a good example. (It does however suck, but for unrelated reasons.)

.. as of yet, promises aside.

Someone should try porting the open source TG clients to the matrix protocol by the way..

You might want to be more specific on the definition of "not sold your soul" when praising a messaging app that holds the encryption keys of their homemade encryption scheme on their closed-source servers.