Bug 1144127 was checked in. This means that starting in Firefox 40, placing add-ons in the distribution/bundles directory will no longer work.

For many years I recommended distribution/bundles as the best place for enterprises to deploy non bootstrapped extensions. It allowed them to make their extensions a part of core Firefox and prevent users from removing them. Unfortunately adware/spyware folks started using this method as well, so we lost this ability. (This is why we can't have nice things.)

So what does this mean going forward?

• You will no longer be able to disable safe mode. You can set the environment variable MOZ_DISABLE_SAFE_MODE_KEY to prevent using the startup shortcut or set MOZ_DISABLE_AUTO_SAFE_MODE to prevent crashes from starting safe mode, but a user will always be able to start Firefox in safe mode from the command line.
• It's much more difficult for you to prevent a user from disabling any extensions you need to add for your company. You'll probably need to do something evil like hide them inside of the add-ons manager. You can contact me if you need code to do that.
• AutoConfig now becomes the preferred method of doing pretty much any Firefox configuration (since you can't place a custom extension into the distribution/bundles directory).

I'm actively working on making the CCK2 work without the distribution directory. The latest beta is here. Obviously some features will be lost st first. I hope to bring as many back as I can. It should be ready by the end of the week I hope.

As a side note, this means that many of my blog posts will have incorrect information. I'm still trying to figure out how to solve that going forward.

https://mike.kaply.com/2015/05/19/distributionbundles-directory-gone-in-firefox-40/