There are lots of elements inside of the window, so this add-on is small example of XUL for those, who just start creation of add-ons.
About Acid2 test: there is a link to local copy of the test on the bottom of the window. Firefox 1.5 and 2 will not pass the test, while Firefox 3 will.
The Hello World extension is an Old Hello World extension for Firefox 3 with Acid2 test added.
Note: The Hello World addon is cross-platform and it works on Mac OS X, Windows and Linux.
