is your default search setting changing to Yahoo, or is it being ignored?
One possible culprit would be an extension. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:
Command+Shift+a (Windows: Ctrl+Shift+a)
"3-bar" menu button (or Tools menu) > Add-ons
In the left column, click Extensions. Then cast a critical eye over the list on the right side. All extensions are optional. If in doubt, Disable. If it's something you don't remember installing, try Remove.