Where are you trying to get rid of Yahoo from? For example:

• Home page
  • See: Startup, home page, tabs, and download settings

• Change default search engine
  • See: Use the Search bar in Firefox and manage search engines

• New Tab page

• Somewhere else?

If Firefox won't let you save changes to these settings:

• Check the Windows Control Panel, Uninstall a Program, and make sure you do not have something called SearchProtect on your system. If you find it, remove it.

If you can successfully change settings during your session, but at the next startup they are undone:

• Check this article for more information: How to fix preferences that won't save

Any luck?

You can use the SearchReset extension to reset some preferences to the default values.

• https://addons.mozilla.org/firefox/addon/searchreset/

Note that the SearchReset extension only runs once and then uninstalls automatically, so it won't show on the "Firefox > Add-ons" page (about:addons).

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

If you do not keep changes after a restart or otherwise have problems with preferences, see:

• http://kb.mozillazine.org/Preferences_not_saved