One of these extensions I came across automatically closed about:extensions every time I opened it to prevent uninstallation. Eventually went through the chrome task manager, killing extensions one by one until I found the right one by trial and error. Very frustrating to debug.
Additionally, JS is a dynamic language so it's difficult to provide adequate automated scanning for malicious intent. Even so, extension authors can be quite clever in how they hide malicious behavior, especially if there's financial reward involved.
WebExtensions (https://wiki.mozilla.org/WebExtensions) add a permission system, have a much smaller attack surface, and should help to alleviate this problem. This combined with automated scanning + human review and
For Chrome, I think Google prefers a combination of automated scanning and quick response, without upfront human review (based on https://static.googleusercontent.com/media/research.google.c...)
[1] http://www.ghacks.net/2013/03/12/mozilla-needs-a-new-audit-p... [2] https://addons.mozilla.org/en-US/firefox/addon/blocksite/rev... [3] https://bugzilla.mozilla.org/show_bug.cgi?id=903799
You could imagine that a malicious company would put through a "clean" version for testing and then once approved swap the script for the one loading malware.