I want to remove the Yahoo search engine but the remove tab is not highlighted or blocked somehow.
Did you read these instructions?
Your Add-ons list in the troubleshooting information doesn't show a Yahoo search engine. This means that you either have an extension that installs this or you have malware.
You can do a malware scan just to be sure.
Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.
- switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
- do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window
If clearing the "Restore Default Search Engines" on the "Settings -> Search" page doesn't work, try to rename/remove search.json.mozlz4 in the Firefox profile folder with Firefox closed to rebuild the search engines registry.
You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).
- Help -> More Troubleshooting Information -> Profile Folder/Directory:
Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
Firefox will rebuild search.json.mozlz4 from the built-in search engines.