You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

After the above, Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh).

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Make sure you know what each one is and what it does.

Firefox Safe mode enabled me to access the Add-on Manager which had been blocked and remove the Yahoo Search extension. The Search Mode is now cleared. Thanks.

Glad to help. Safe Surfing.