I had this problem in my windows 7 after updating firefox to latest version. I found that people also get this problem after installing the Microsoft Office.

Here is the trick.

Go to registry editor,

Start -> Run -> regedit

Search for HKEY_CLASSES_ROOT\FirefoxHTML\ShellEx\IconHandler

Delete the key IconHandler, and try changing the default program for your html file.

If it dont work,

delete whole FirefoxHTML from the registry, This will fix it.