The current Firefox extension platform is a mess. There are XUL add-ons, Jetpack add-ons, and SDK add-ons, nothing is fully compatible with E10s, and to say the security features are lacking would be an understatement.

The Chrome platform on the other hand was designed to be multi-process from the ground up, has some solid security research behind it (like actual academic studies), and generally just makes sense. The Chrome extension system (or something very similar to it) is also already supported in a bunch of other browsers, so why would Mozilla not use it in theirs?

E: typo