This will be caused by an add-on. Check any Yahoo related add-ons to see if they have added this. The freeze net assistant add-on is another one that will cause Yahoo search to be displayed on new tabs. If you have that add-on you can uninstall it.

If you can not find what add-on is causing this, the procedure in the following link will help you identify the culprit - https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Thank you that worked perfectly, I just had to get rid of the freeze net assistant add-on.

ok I'll try.. tnks!

I could not find anything in my add-ons. What I did find was an installed program called 'Netassistant for Firefox'. It was listed in the registry as associated with "freeze.com" . I uninstalled it and tab now opens with a blank page

Firefox made a change in the last few updates. Firefox is now forcing yahoo. It's not easy to remove it but it can be done.

Also, Firefox don't want you to have the file menu. Again, you can have it back. It's not easy but it can be done.

To show the Menu Bar, press the Alt key in Firefox. View menu, Toolbars, Menu Bar - to have it always shown.