I use mobile firefox as well and it switched to yahoo as well. i could change it back without any issue. i think they changed defaults - but if you had put your own engine yourself it didnt.

Yes and no. Changeable search engines were obviously a prerequisite for this kind of deal, and Yahoo was one of the first ones to offer the needed APIs (and IIRC, over HTTPS which was required by Mozilla).

Changing search engines can be done via Customize->Search->Installed search engines. Click the one you want and you'll get a popup offering to remove it or make it the default.