A big problem for a lot of Firefox users is addons that are side-loaded into a profile. Like when you download some junk and it comes with an installer that injects an addon into your profile. This is a good expose on what even a seemingly legitimate site like download.com does: http://www.howtogeek.com/198622/heres-what-happens-when-you-... (and several of the malware examples in that article are side-loaded addons).
Unfortunately hoops aren't enough to fix this problem, because hoops generally involve some flag that you've gone through the hoop, and the installer can easily fake that.