Which page is overriding your homepage?
Might be caused by Malware.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

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Does it appear every time you open a new window (Ctrl+n), or only when you start Firefox using its program icon?

If it's only when using the program icon, the shortcut might have been hacked.

right-click the shortcut > Properties > Shortcut tab

For 64-bit Windows 7, the Target line should be exactly the following:

"C:\Program Files (x86)\Mozilla Firefox\firefox.exe"

If there is an address after that, delete it.

What is the URL of that home page?

You can use the SearchReset extension to reset some preferences to the default values.

• https://addons.mozilla.org/firefox/addon/searchreset/

Note that the SearchReset extension only runs once and then uninstalls automatically, so it won't show on the "Firefox > Add-ons" page (about:addons).

If you do not keep changes after a restart or otherwise have problems with preferences, see:

• http://kb.mozillazine.org/Preferences_not_saved