If other major browsers are cutting off critical functionality for it, the concern may be moot.

As long as Mozilla aren't themselves in ad sales/brokerage I wouldn't be worried about a browser shipping an ad-blocker, as far as incentives go. Google doing it, that'd be concerning. Mozilla? Good.

[EDIT] though actually this is another case of their relationship with Google being kinda crippling, since that does introduce a conflict of interest... which is part of why Google does it, I'm sure.