> Are (updates to) extensions from the Chrome and Firefox store usually vetted before publication?

Mozilla does not manually review most extensions (only extensions which Mozilla recommended are manually reviewed: https://support.mozilla.org/en-US/kb/add-on-badges).

Chrome's policy is extensions are "reviewed periodically for compliance", but is unclear on how frequent is this periodic review (https://developer.chrome.com/docs/webstore/review-process).