if Mozilla stopped maintaining Firefox, almost all of its forks would die.
That's an assumption. You're severely underestimating the amount of work open source maintainers put into those things.
Would these people pay [...] Canonical for their work?
Plenty of nice distribs exist without being maintained by a corporation. Good on them, for getting to have a bunch of money of the opensource, but it's not a necessary model for the opensource projects to survive.
I see a lot of people shit on Firefox for integrating LLM-related features
And that's good, as it should be. They crammed this bullshit that nobody asked for, and reluctantly gave a button to opt out a year later, and it's still on by default. This is bad behavior regardless of your stance on it.