Amongst other things it could be

• You have simply selected Yahoo in the Search bar.
  • Choose another option Use popular search suggestions in the Firefox Search bar
    Note recent Firefox versions use the selected search engine for all the searches including the addressbar and on web searches offered from right clicking highlighted text in a webpage.
• A malware problem.
  • see Troubleshoot Firefox issues caused by malware_5-reinstall-firefox
  • And use the search reset addon afterwards https://addons.mozilla.org/firefox/addon/searchreset/
  • and Remove a toolbar that has taken over your Firefox search or home page
• It may not be an official copy of Firefox, it may be a branded version,
  • Reinstall Firefox using an official copy from an official site. Note you will need to delete the Firefox program Files. (NOT the profile files, and do NOT use any option offering to remove personal information or you will destroy bookmarks, passwords and settings.)
  • Download from http://www.mozilla.org/en-US/firefox/all/
  • See Troubleshoot and diagnose Firefox problems

Please post back with what you tried and how you got on. I can offer further advice if necessary once I know more about the problem.

It may help to have full troubleshooting information

It helps to have some more non-personal information Please do the following:

1. Click the menu button , click help and select Troubleshooting Information.
2. Now, a new tab containing your troubleshooting information should open.
3. At the top of the page, you should see a button that says "Copy text to clipboard". Click it.
4. Now, go back to your forum post and click inside the reply box. Press Ctrl+V to paste all the information you copied into the forum post.

If you need further information about the Troubleshooting information page, please read the article Use the Troubleshooting Information page to help fix Firefox issues.

Thanks in advance for your help!

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You can check for issues caused by plugins (plugins are not affected by Firefox Safe Mode)

• https://support.mozilla.org/kb/Troubleshooting+plugins

When I follow a link from Google, the correct page will appear but is immediately overridden by a yahoo search. It will not let me return to the correct page.