StatCounter has also run a special report on US search engine usage by Firefox users only. Yahoo-on-Firefox usage in the US increased from 9.9% in November 2014 to 28.3% in January. Over the same period Google-on-Firefox usage in the US fell from 81.9% to 63.9%.
Just goes to show how many people never change the default settings in apps. This kind of change is pretty extreme and I'm willing to bet Yahoo will take more share amongst Firefox users in the months to come because plenty of people are usually slow to updating to the latest versions of their web browser too.
Worth noting: I discussed this with my co-hosts on the first episode of the Technically Correct Podcast