I'd swear this happened already... I updated Firefox on an old Android tablet yesterday and the default search was already switched to Yahoo. Edit: ah, now that you mention it, it was a Beta version.
Firefox for Android has been testing Yahoo as the default search engine for a few months. Mozilla's search deal with Google did not cover mobile browsers. Firefox OS uses Bing.
Firefox OS does not use Bing. It used Bing a long time ago but all consumer devices are set to Google these days.
Pre-release versions (Beta, Aurrora, Nightly) on Android have been running experiments with Yahoo for a while.