From MozillaZine Knowledge Base

If addresses (URLs) that are typed into Internet Explorer's Address Bar open Firefox, the problem can be caused by having IE6 and IE7 Beta installed side by side. [1] or can possibly occur after a failed IE7 installation. According to this Microsoft Internet Explorer Weblog the recommended solution is to uninstall and reinstall IE7.

If you wish to continue using IE6, you can edit the registry if you are an experienced user, to remove the HKEY_CLASSES_ROOT\CLSID\ {c90250f3-4d7d-4991-9b69-a5c5bc1c2ae6} registry key.

Warning: Editing the registry is a risky undertaking. If you make a mistake and delete the wrong key you can damage your system and make it unusable. Proceed at your own risk!

1. Open regedit
2. Navigate to HKEY_CLASSES_ROOT\CLSID\{c90250f3-4d7d-4991-9b69-a5c5bc1c2ae6}
3. Right click on {c90250f3-4d7d-4991-9b69-a5c5bc1c2ae6}
4. Press Delete

For more information see this blog.