Society doesn't get improved by doing incremental work on a browser, and Mozilla's mission is to improve society

Yes it does. Having a browser that truly has the user's back, without always trying to compromise the user's interests in favor of advertisers - that would be a benefit to society.

I wish Firefox would be that browser.

Possibly, but that's an absurdly overbroad mission.

Organizations with clear, focused missions are much more likely to be able to achieve them than organizations that want to be everything to everybody.

"Make and maintain Firefox as the best browser for people who care about internet freedom, privacy, and extensibility" would be a perfectly reasonable mission.