Audone, you've somehow gotten a Yahoo extension installed. Some not-so-nice apps install them without telling you. Have you installed anything new lately?
At any rate, you can fix it pretty easily:
In Firefox, go to Tools->Add-ons->Extensions and see if there is anything Yahoo in there and remove it. If there is nothing there, then follow the instructions in the first link below for looking at what is listed in about:config (put that in the address bar, hit return, then search for Yahoo using the search window). You will get a warning message when you put about:config in the address bar - uncheck the option to show it every time and dismiss it.
If there was a Yahoo extension installed, once you get rid of it, you should be able to reset your search preferences by clicking on the dropdown menu (gray arrow head) beside what may still say "Yahoo" in your search window and then either select Google, if that's in the list, or click on "Manage Search Engines" and proceed from there, either adding what you'd like to use or selecting one already in the list.
https://support.mozilla.org/en-US/questions/995599Also read this, perhaps:
https://support.mozilla.org/en-US/questions/910589.
