Maybe it's obvious to you, but it's not clear from the question.

The location bar is supposed to be blank if you have a blank page as your home page. If you have a non-blank page as your home page, that page's URL is shown in the location bar.

You could also have set Firefox to show the pages from the last time, in this case instead of your home page the pages from the last session are loaded.

Did you check How to set the home page?

If the URL doesn't correspond to the page shown, it would be helpful if you copied the URL and pasted it here (and attached a screenshot).

Which URL do you see in the location bar?

Do a malware check with a few malware scan programs. You need to use all programs because each detects different malware.

http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware http://www.superantispyware.com/ - SuperAntispyware http://www.safer-networking.org/en/index.html - Spybot Search & Destroy http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page .......... See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked If you can't fix it with the above listed scanners then you need to ask advise on one of the forums that specialize in malware removal mentioned in the Popups_not_blocked article.

Below is URL I see:

http://my.freeze.com/?AcquisitionID=7861e862-7fe8-477e-ac14-52526c7e950a&s=&ipc=&vintage=20090938&defaultBrowserId=9&productId=2138&vendorId=3852&offerId=4677

cor-el was right, it's malware (you can see it by searching Google for "my.freeze.com"). You should follow his suggestions now.