Here's a surprise. Like most Firefox users, I've added some extensions. They come from the Mozilla site, and I've always assumed that they, like Firefox itself, are open source.

Well, it turns out, that some of them aren't. According to Command Line Warriors, the popular StumbleUpon extension, and probably others, actually have a proprietary licence.

My favourite extensions, the fabulous Adblock Plus and excellent Foxmarks are open source, it turns out. But StumbleUpon, it turns out, has a not-very-well-displayed licence, which denies you the right to reverse-engineer, or make derivative works from the tool-bar.

(It also says the licence is only for non-commercial use, so I guess I can't use it since I surf, read and write for a living.)

This is not a huge concern. Firefox is still open source, and the platform on which these extensions are built dose not (I presume) require all extensions to be open source.

These licences will be hell to enforce, since no-one clicks an "I Accept" button to install an extension, and since all Firefox extensions include their source code in a zip file.

But it's another illustration that things are more complex than you expect. Open source software (such as Firefox) can support proprietary extensions, just as easily as proprietary software (such as Windows) can support open source extensions (such as Firefox).