That's an interesting idea. I, too, would happily contribute to a funding pool to have a competent human analyze the extensions that I'm using. One would expect this to (loosely) allocate money to checking the extensions that are most used, so the ecosystem would benefit even for people who weren't contributing.

And corporations would pay a lot to have extensions security reviewed if they had a business use case.

Very nice idea. As an ISV with a freemium open-source extension, I would happily pay for an official security review that I could present to our users. Our extension is secure by design but having a 3rd-party expert (e. g. Google or Firefox reviewer) confirm this and get a "Reviewed" badge would be great. The drawback is that this could slow down extension updates, as you do not want to loose this badge with the next update. But that is manageable.