Mozilla yes - there's a delay when publishing whilst an actual human reviews the changes. For Google, updates are instantly published with, as far as I can tell, no kind of audit.
The Mozilla one is great, they insist on reproducible builds and do a thorough review. Although they can't catch everything, I would pick FF over Chrome all day for this reason alone.
Are you sure about that? AFAIK there is just an automatic validation system. I am an extension author myself and this is from a recent update approval email:
"This version has been screened and approved for the public. Keep in mind that other reviewers may look into this version in the future and determine that it requires changes or should be taken down. In that case, you will be notified again with details and next steps."
Perhaps this also depends on the number of users...
