Hi Keith, could you check these articles:

(1) How to select your default search engine:

Change your default search settings in Firefox

(2) If searches are mysteriously hijacked away from your default search engine:

This is most commonly caused by a rogue add-on. Maybe it was for privacy or maybe it just makes money by diverting your searches. Either way, you can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• click the Extensions button (puzzle piece) on the toolbar > Manage Extensions
• 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. On the right side, find the "Manage Your Extensions" heading.

If there is at least one extension before the next heading -- "Recommended Extensions" -- please continue:

Then cast a critical eye over the list below that heading. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove). For your privacy and security, don't let mystery programs linger here.

Any improvement?

Note: Sometimes extensions can frustrate access to this page. In that case, first restart Firefox in its Troubleshot Mode and then try again. See: Diagnose Firefox issues using Troubleshoot Mode -- Open, not Refresh.

I agree that it has to be an extension you installed or updated recently considering Mozilla removed the Yahoo search engine from (desktop) Firefox starting Firefox 57.0