There's an outfit called WIPS which buys up abandoned Firefox add-ons and puts adware and spyware in them.[1] BlockSite [2] is an example. This was approved by Mozilla AMO.[3]

[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