Malware extensions in firefox extensions is much more common than you think. And they use really sophisticated methods to evade detection and to get the extension published on the extension store. Article deep dives into one of these, and explains how it works by referencing code examples.

I've thought browser extensions and plugins were bad news since '98 or so when my boomer relatives were browsing on 640x480 screens that only had about 80px or so vertical space for browsing the web because their browsers were stuffed with worthless toolbars. They thought it was normal the same way they were about to think it was normal to watch SD television stretched out on an HD screen.