I understand that Yahoo searches when Google Search is selected as the default search engine. Can you please share where you are searching from?

If this is happening from the url bar, try a Firefox Reset. If this is happening from a homepage, you can change the homepage to a different home page: How to set the home page

Also make sure that there is not a toolbar installed that might be changing the search preference as well.

Please post back with your results.

Following up on one of those points, one possible culprit would be an extension. If you prefer a little troubleshooting before using the Refresh feature...

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.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

I had the same problem, went to my add-ons and found Search for Firefox add-on I never installed. I removed it, and now I'm reunited with Google.