There are couple changes in Firefox 40 that folks need to be aware of. One I've mentioned, one I just found out about.

1. The distribution/bundles directory is no longer used. As I said in this post, you must get the latest CCK2 and rebuild your AutoConfig distribution for use on Firefox 40. The new distribution you build will work as far back as Firefox 31. Unfortunately, the ability to disable safe mode won't work on Firefox 40 and beyond.
2. The searchplugins directory is no longer in browser/searchplugins; it's integrated into the the product. If you were using this directory to add or remove engines, it won't work. You can either use the CCK2 or put the engine XML files in distribution/searchplugins/common. See this post.

If you have additional problems, please let me know.

https://mike.kaply.com/2015/08/13/firefox-40-breaking-changes-for-enterprise/