MozLab
MozRepl and MozUnit (formerly bundled as MozLab) will hopefully make your experience of developing AJAX and Mozilla applications suck less. They're packaged as extensions for Firefox/Thunderbird/XULRunner and released as open source. Have a look at the videos for some of the cool things you can do with them.
MozRepl
Connect to Firefox and other Mozilla apps, explore and modify them from the inside, while they're running.
Execute Javascript, play with browser GUI, sneak into HTML pages, examine functions and variables, redefine them on the fly, hot-fix bugs, ...
MozUnit
Develop test-first style or just test against regressions: MozUnit provides framework, test runner, source browser, and API hooks for personalized reports.
Community
Join the mailing list.

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