In Firefox version 34 and higher, the default search engine has changed from Google to Yahoo. This was due to a new agreement between Yahoo and Firefox. If you would prefer to have Google back as your default search engine, here is how to do that.
1a) If you have the menu bar enabled, go up to Tools and select Options.
1b) If you do not have the menu bar enabled, you can go to the 3 bar menu icon on the top right side and select Options.
2) Select the Search tab.
3) Left click the dropdown box in the Default Search Engine section and choose Google (or whatever search engine you want to use as default).
4) Left click OK to save.
If you have any questions about this tutorial, please post them in the comment section below. Thanks!