Just a couple of tips for Firefox extension developers, hard earned after many hours of head scratching. Not adhering to either tips will confuse Firefox and XPCOM component will fail to load.

XPCOM components get loaded before chromes are loaded.

[Update: The most common problem related to this is Components.utils.import call fails during launch with NS_ERROR_FAILURE exception. To fix, wait until app-startup notification is received before importing javascript modules.]

This means anything defined in chrome.manifest won’t be available until “app-startup” event is observed. Note that Resource URI scheme “resource://” introduced in Firefox 3 uses resource directives in chrome.manifest which means you should defer Components.utils.import calls until “app-startup“.

XPCOM components implemented using Javascript should be defined as a pure object, not function.

So it should look something like this:

var MyServiceModule = {
  registerSelf: function(compMgr, fileSpec, location, type) {
    ..
  },
  ..
};