Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

Reset the browser.startup.homepage_override.mstone pref via the right-click context menu to the default on the about:config page and close and restart Firefox to restore the about:home page to its default.

• Open the about:config page via the location bar
• Type in the Filter bar: mstone
• Right-click the browser.startup.homepage_override.mstone line and select: Reset
• Close and restart Firefox

It is very simple. Yahoo uses Microsoft .NET FRAMEWORK Assistant to hijack your browser. Go to the upper left hand corner (Firefox 10 - it might be elsewhere on older versions) and click on the drop down arrow. On the right hand side, click on "Add-Ons", and a list will appear. Locate .NET Framework Assistant. On the right side, click on "disable" and then either click on the small line that says "restart", or simply close Firefox by clicking on the red "X" in the upper right hand corner of your screen. Reopen Firefox and -Voilà, your problem is solved. As far as I am concerned, Yahoo has created a virus when they hijack your preferred browser - a good reason to dump Yahoo for any other use. On the other hand, Microsoft is not innocent in this endeavor. If you want to get rid of the real problem. Dump windows and switch to Ubuntu Linux. You can find it free on the internet.