The documentation is a mess. Right now there exist 3 different ways of developing your extensions. Even if you decide you are going to use the official Addon SDK it is not clear if you should be using the CFX tool like described in their main repository[1] or JPM[2] which according to this blog post[3] is now the official tool. If you decide to go with the new tool you will find out that there is no documentation how the import system works, except another blog post[4] that just says they are now using npm. Good luck with figuring out all the details.
But I think the main problem with Firefox is that once the Addon is inside the browser there is no sandboxing. The Addon has the same privileges as the Firefox process, including the potential to modify other Addons at runtime. That's why the review process is taking such a long time, they basically need to hand check all of your source code to catch if you are doing something nasty. If you don't ask in Chrome for some permission you will just not have access to this API, but not in Firefox. No permissions, always full access. This is an enormous security risk, because it is impossible to check all corner cases of big Addons and be sure that the Addon will not get code from outside and just eval it.
[1] https://github.com/mozilla/addon-sdk [2] https://github.com/mozilla/jpm [3] https://blog.mozilla.org/addons/2014/08/19/announcing-add-on... [4] http://work.erikvold.com/jetpack/2014/08/07/cfx-to-jpm.html