SearchYa may have "piggy-backed" onto your system when you installed some third-party software on your system; always look for an opportunity to not install extra items or look for a custom install option when installing software or software updates.
Remove SearchYa Toolbar (if you have it installed):
I had the same problem. Everytime I opened a new tab it got SearchYa as search engine. The about:config solution did not help; it has reseted as soon as I closed about:config tab.
Finally I got rid of it by selecting: Firefox -> "Help" then "Troubleshooting Information" then click on "Reset Firefox" button. Firefox closes and resets. When it starts again all the features are reseted and the problem solved. It had kept all my bookmark and toolbar settings.