According to [0], Mozilla requires all extensions to have a source in human-readable format and runs a test suite on them. They mention "code review" there, but don't say whether it's manual or automatic. I'd love to hear about it from someone who has some experience with the process.
For my extension the review was automatic, and flags things like direct html editing. I only have a few hundred users though, so I'm not sure at what point they decide to do manual reviews.
[0] https://extensionworkshop.com/documentation/publish/submitti...