What is your search engine homepage and what is your default search engine? There's alot of missing information here for anyone to know what is going on here.

https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox https://support.mozilla.org/en-US/kb/search-firefox-address-bar https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox

Does it still happen in Troubleshoot Mode?

How do I fix this? I like to use Google for my searches and Yahoo takes over every time. I can switch it, but it's such a nuisance and time-waster to have to keep switching it back to Google. What can I do?

Google search engine in the search bar or by way of Google website?

Firefox has not come with anything Yahoo for over five years now (as you can see in /forums/contributors/712925) as perhaps this Yahoo search you are seeing was added by a recent Extension install or update in Firefox? as some have done this in the past.