Just a reminder to myself, because I want to try that extension:
What then, is Mochitest-Lite?
Mochitest, as it is used at Mozilla, requires Python to drive the command line interface, and automate the testing. Python is a very powerful tool for this purpose because, along with automation, it can overwrite and then rewrite configuration files like manifests and preferences, all while having a minimal impact on the application being tested. Mochitest also uses a client-server architecture to serve test documents to the application being tested. On top of that, an application needs to be built with the Mozilla build system to work with Mochitest.
This makes sense for large applications like Firefox, but what about a unit testing framework for developers of small XULrunner applications and extensions for existing Mozilla applications? These developers have different requirements. Mochitest-Lite is a way to get the same test writing tools and automation without the overhead of running Mochitest the same way it is used to test Mozilla code.