Hi Kristin, what is the extent of the hijack? For example:

• Yahoo has taken over your home page and/or new tab page?
• Searches you enter into Firefox's address bar say Firefox will "Search with Google" but you get results on Yahoo?
• Searches you enter into the search box on Google's own home page give you results on Yahoo?

Could you test in Firefox's Safe/Troubleshoot Mode? In that mode, Firefox temporarily deactivates extensions, hardware acceleration, any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem.

You can restart Firefox in Safe/Troubleshoot Mode using either:

• "3-bar" menu button > Help > Troubleshoot Mode... (before Fx88: Restart with Add-ons Disabled)
• (menu bar) Help menu > Troubleshoot Mode... (before Fx88: Restart with Add-ons Disabled)

and OK the restart. A small dialog should appear. Click the Open button (before Fx88: "Start in Safe Mode" button).

Any difference?

Type about:addons<enter> in the address box 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 remove SafeSearch, SearchAssist Incognito, FileConverter Addon, SearchLock, DiscreteSearch, WebSearch, Findwide Search Engine, securedsearch

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.

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 free to use malware scanners. Each works differently. If one program misses something, another may pick it up.