Why is Google, a company with gross revenue of much more than that, not doing it at all?
Obviously, the Mozilla Corporation's only stake holder is the non-profit Mozilla Foundation, so they can't pay out much of their revenue to anyone other than their employees (and to those in wages that are comparable with the rest of the industry; they have to publish their finances for that), so they certainly can afford to spend more money on things like that, but they're still trying to avoid spending huge amounts of money on something that can be done just as well in 99% of cases at a fifth of the cost.
As far as I know, they have a system where at least two independent volunteers have to approve an extension and some employees will occasionally check up on things, too, and can revoke volunteers that make mistakes too often or are plain malicious.
So, most of the time, this system will fail on the side of things being disapproved that should have been approved. It will much less often be the case that for example malware will get approved (especially since they also have automated malware scans, which is something that Google has, too).
Obviously, the Mozilla Corporation's only stake holder is the non-profit Mozilla Foundation, so they can't pay out much of their revenue to anyone other than their employees (and to those in wages that are comparable with the rest of the industry; they have to publish their finances for that), so they certainly can afford to spend more money on things like that, but they're still trying to avoid spending huge amounts of money on something that can be done just as well in 99% of cases at a fifth of the cost.
As far as I know, they have a system where at least two independent volunteers have to approve an extension and some employees will occasionally check up on things, too, and can revoke volunteers that make mistakes too often or are plain malicious.
So, most of the time, this system will fail on the side of things being disapproved that should have been approved. It will much less often be the case that for example malware will get approved (especially since they also have automated malware scans, which is something that Google has, too).