Firefox's 34th edition is out and one of the most noticeable things is the search default is now Yahoo. Apparently they left Google over money opportunities but I also wonder if it didn't have to do with how Google's search results are changing so much that you can no longer find what you are looking for! I admit I still use Google but I do find myself still having to use Bing and Yahoo to find more because I just don't hit the right sites on the first try like I used to do with Google.
It will be no surprise if money was the reason but it could be due to many factors including what you mentioned. The top heads at Firefox really have not officially said the reason so its all speculation. It did cause Google Shares to take a dive at the start of the year.