Personally, I'm fine paying for some ad-free content. But Mozilla wants much of the internet to be free (as in free beer) and open, so an internet which replaces all advertising with paywalls is not their vision. Advertising without tracking, surveillance and shady data brokers sounds better than advertising with tracking, surveillance and shady data brokers.
I'd love for their to be another driver, but so far nobody has come with a broadly applicable revenue model other that paying for content or viewing ads. Suggesting that "just delete advertising" without providing an alternative is feasible is patently false and everything that follows is compromised.
Perhaps one day we will live in a post-capitalist utopia, but in the meantime making online advertising less shitty would be nice.