Mozilla prohibits connections to the internet, which aren't necessary for the advertised functionality of an extension. So, these are rather "Chrome extensions" we're talking about...

I wonder how easy it would be to make an extension and fake it's popularity? Make make it intentionally broken or something, so users immediately uninstall it too.

Sounds like an easy $10k, assuming the scammers would actually pay.

Don't extensions get reviewed by the various stores? I'd imagine an automated check could catch malicious integrations like that.

Maybe not right away, but once they catch wind of one shady extension they could just search the store for any other ones.