This is likely to have been modified by an add-on, the built-on option for Firefox will only use a blank page. Check the settings for your add-ons to see if one has such a setting. You can also follow the procedure in the troubleshooting extensions and themes article to identify the culprit.

I had the same problem. I found the solution in this article:

http://medicine-opera.com/2010/03/how-to-disable-yahoos-theft-of-firefoxs-open-a-new-tab/

Hope this helps.

Here’s the easy, get rid of it for good fix: It’s done within Firefox. Enter [about:config] in the url address box. Click “I’ll be careful. I promise”. In the <filter> field, type “extensions.netassistant” (no quotes). Right click on the field: “extensions.netassistant.tabsearch.url”, click on , either delete its string or enter your own. I’ve entered “https://encrypted.google.com” (this is a beta encrypted search by Google) just as a personal satisfaction against Yahoo. Do the same for:”extensions.netassistant.url” field. NOTE: If you haven’t modified your Firefox configuration before and you don’t feel comfortable in what you are about to do, do a backup of your preferences. The backup procedure can be found at: http://support.mozilla.com/en-US/kb/backing%20up%20your%20information. Or just do a Google search for “backup Firefox pref.js”.

UPDATE:(2/26/2011 5:04PM CST) After all that tinkering with [about:config], (which did work for me), I've found an even simpler answer: -Close all browsers. -Open [control panel]. -Open [add/remove programs] -Remove "net assistant for Firefox" (may be called: "net assistant for Mozilla") -That's it. Restart Firefox and see if it worked.

Thanks FlipFlops2001! I used the update/simple fix, and it worked perfect!!

it was very helpful,thanks a lot