A month ago, Kottke was wondering whether Mozilla Foundation has the vision to make Firefox the most important piece of software of this decade (as an open application platform). I offered my opinion on Paolo's blog: XUL+Javascript is starting to get more and more interesting; however, Javascript is not (yet) well suited for writing large applications. I would love to see a Python or a Ruby engine integrated within Firefox or XUL Runner, that would really be a viable foundation for open, cross-platform network-based applications. Being able to exploit the client (as opposite to heavily rely on the server side) while still being network-based is still an unreolved issue for me.

Today, Mozillazine mentions Brendan Eich's work. Brendan is developing the infrastructure which will allow Python to be used for XUL scripting:

Support for Python in XUL will land in the Mozilla 1.9 timeframe and is expected to be used primarily by developers of extensions and standalone XULRunner applications.

Work is currently ongoing to allow languages other than JavaScript to be used for DOM scripting, which is a necessary step to enable Python support to be implemented. In theory, this will also allow support for other scripting languages to be added to the Mozilla framework. However, there are no plans to support any languages other than JavaScript in webpages.

Although future releases of Mozilla applications such as Mozilla Firefox and Mozilla Thunderbird will ship with support for Python XUL scripting, a Python interpreter will not be included. This is expected to mostly be an issue on Windows (Mac OS X and most Linux distributions already include a Python environment) but Brendan expects this problem to be solved soon.

The Mozilla Foundation has an (old) document on this topic: Roadmap for Language-Agnostic Scripting Support.