This helps you to test your addons. This is based on MozLab, and this is compatible to it. You can run test cases written for MozUnit, on this tool.
You can run not only unit tests for your custom classes, but also functional tests of actual features. "yield" statement is avalable in test cases like as "wait", so you can wait Firefox's actions (ex. page loadings).
And, some useful helper-functions are available, like:
* Open a Firefox window for testing.
* Import (include) external scripts.
* Send click event to elements.
To learn testcase syntaxes, see: http://www.clear-code.com/software/uxu/index.html.en