You should be able to find it via Tools | Add-ons | Extensions where you can remove it.

i have 4 extensions. avg security toolbar google toolbar for firefox microsoft.net framework assistant and skype extension.

should i remove one of these?

You best bet would be to run Firefox in Diagnose Firefox issues using Troubleshoot Mode. This will disable all your extensions temporarily. If it works without loading Yahoo, then it's probably either the AVG toolbar, or Skype which is the culprit.

However, in this particular case, it sounds more or less like a browser hijack. What I suggest you do if running in Safe Mode doesn't fix the problem is the following.

• In the location bar, type about:config and hit Enter.
• In the filter at the top, type: keyword.URL
• Double click it and remove whatever's in there. You'll see it revert to "Default" and "String".
• Go to File | Exit
• Restart Firefox, go to the site you want to set as your homepage.
• Go to Tools | Options | General.
• Make sure it says "Show My Homepage" in the first dropdown menu.
• Click the button called "Use Current Pages" to set the homepage to the one you have on the screen.