Please check in Add/Remove Programs in the Control Panel and see if you have any Yahoo tool Bars listed. If so remove.

You can go into Firefox Tool Bar Menu --> Tools --> Options --> Search And Highlight any search engine you do not want and click remove.

Please Note : that other programs like Flash can install and change your search engines. Always ready carefully before installing.

Please follow : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware and https://www.malwarebytes.com/adwcleaner/ and if found and can ot remove or need further help removing things https://forums.malwarebytes.com/topic/9573-im-infected-what-do-i-do-now/

Please let us know if this solved your issue or if need further assistance.

In addition to the above:

If you get hijacked away even when you type a search directly into the Google home page, it's most likely an unwanted extension.

You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then cast a critical eye over the list on the right side. Any system extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything that contains the word "search" or looks suspicious? Anything with the word "incognito"? If in doubt, disable.

Sometimes a link will appear above a disabled extension to restart Firefox before the change takes effect. You can complete your work on the tab and click one of the links as the last step.

Any improvement?