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

To get the Welcome to Yahoo Toolbar tab to stop coming up I have to disable the toolbar. What's the use? Thanks for the replies anyway.

That is a question to be asked to Yahoo toolbar maker I guess, Darrell have you searched in the toolbar setting for some option that makes the new tab open.

Did you try to delete or rename the prefs.js file to check for issues with the preferences?

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

Resetting Firefox solved the problem. The Welcome to Yahoo Toolbar tab only opened once after resetting and hasn't happened since. Thanks to all who replied.