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. Remove “ Yahoo “ Anything. Also look for SafeSearch or SearchAssist Incognito.

Now do the same in the computer’s Programs Folder.

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the Applications folder. Linux: Check your user manual.

After you have done as mentioned above and removed anything that you don't recognize and looks suspicious:   most malware will be 'hiding' somewhere , so let's run malware scans :

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.
They all work differently   -   what one program doesn't pick up, the other might.