It's not really a "both sides" thing given the scale difference between what Google is up to and that.

No one is forcing Mozilla’s hand (afaik) and no one is actually thinking “if Firefox had a chatbot that might convince me to move over”. Youre right that it is not a “both sides” thing, its a “Mozilla thing”. Through and through.

Some people are thinking "Firefox doesn't have a chatbot so I'll use chrome" though

Are any of those people not investors?

Really? How many people can that possibly be, and how many of those would a reasonably sane developer actually want?

If your competition is attracting the worst kind of users, do you really want to get in their way?

Yes you want more users not less. This is business 101. Even business 000. Customers good.

Business 101 is to make sales! Users sometimes represent sales, but not always. That may be business 201 though.

This is about market share of a nonprofit - it's users.

Precisely their point: users don’t necessarily mean sales, but they MIGHT bring in donors.