Writing a Firefox plugin for parsing a custom client-side language

If I've understood what you'd like to do, you'll need to write a Gecko plugin. Via a plugin, you will be able to register your own MIME type and then manipulate Javascript & the DOM.

This means you would need to include an <object /> or <embed /> tag on the page to load your plugin, but you could then look for <script type="application/x-yourtype" />, grab the innerText of that script tag and parse it using your plugin.

As Nickolay has suggested, the alternative is to use whatever the browser currently supports or create a custom build of the browser. Daniel Spiewak's suggestion to use a Java applet (or a Flash applet would also work) is also valid.

The information you're after is available on Mozilla's developer website:

• Gecko Plugin API Reference
• Plug-in Basics

An interesting idea. Note that you don't actually need to write a browser-specific plugin to do this. Some people have experimented with using JRuby in an Applet to execute code embedded within <script type="text/ruby">. Such a solution may be slower on startup (due to the overhead of loading an entire JVM instance), but it will be much more flexible in the long run (cross-browser). Besides, it's a bit easier to build a custom language interpreter in a JVM language than it is to try to shoe-horn it into Gecko.

@Nathan de Vries: no, actually, NPAPI plugins you suggested don't let one implement support for <script type=...>.

OP: this is not easy, but look for PyDOM and PyXPCOM - language bindings for Python. The former does exactly what you asked for - for Python, but I'm unsure about its current status. In any case, it's very likely that you need to create your own build of Firefox to support additional script types.