I would pay for a service that reviewed the source code of my extensions (and other installed software) and stamped each specific version as being OK. Then I'd configure my browser not to update an extension to a new version until the extension-verification service had read through the code of the update and okayed it.

Granted, such a service wouldn't have the resources to review all extensions, but it could probably handle vetting the most popular and updates to those popular extensions. I can even imagine some kind of market that would let a group of people get this service to begin vetting a new extension.