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.