In 2021, there is not a chance in hell that any significant number of people are going to pay money for their web browser. At best (and this is very optimistic) it might make a few million dollars from dedicated HN-types, which is nowhere close to what Google gives them.
I actually would pay money for a Mozilla-branded email service, but running an email service isn't a walk in the park, and I expect all of the typical people would be complaining about Mozilla spending time and money on something that isn't Firefox, regardless of whether it's profitable (see also: all the complaining about Pocket).
Honestly, I wish that Mozilla had acquired Scroll (rather than Twitter) and started pushing it harder. That would be very in-line with their goals as an organization, and might eventually become a significant revenue source.
If https://servo.org/ is any measure. Probably not :(
People don't care about browsers until it's too late.
MDN is its only gem left outside of Firefox itself, it's good they're doing this though, but only if all proceeds go primarily towards MDN itself. I can foresee people unsubscribing over funding issues if they find out Mozilla foundation is just pocketing profits from this for things outside of MDN.