I personally think that Firefox Pro could offer a shiny icon that says "Pro" in it, and the satisfaction of knowing that I supported a project I care about.

For an annual license, perhaps a sheet of stickers?

Cloud sync of plugins/tabs. Pocket, filesharing between devices, their VPN nobody uses, Firefox monitor integration, earlier updates (i.e. you can still get the latest build by building from source, but prebuilt libraries first get supplied to Firefox pro),... Basically all the stuff they have acquired over the years, with few users but large support requirements. These are just the ideas I came up with in the spot

This.

A promise to not put ads anywhere

Don't nerf regular Firefox

Let's take YouTube Premium as an example to set a baseline. About 3% of users convert (50M vs 1.5B), for basically the same benefit.

Firefox starts with 200M users now and if 3% get Pro that is 6M users paying $5/month (?) for $30M/month and $360M/year in revenue. That is almost exactly that same as what Firefox makes from Google. The added benefit is that this change could transition it into a user-centric mindset.

Do you agree with the math?