Mac OS X Snow Leopard 10.6
When you have Kompozer opened and that you double click on a html file's icon, the file is not opened by your default browser (firefox) but it's open a new empty window of Kompozer. That is because Firefox and Kompozer have the same CFBundleSignature in the info.plist file.
A file is attached to the topic, with the right value:
<!-- you can change this value, but it mustn't be MOZB, it's firefox's signature-->