Rated 5 out of 5 stars

Very well written, 5 stars! This extension helped me figure out XPCOM much faster than the official tutorials. One observation though, you shouldn't use the Firefox GUID of ec8030f7-c20a-464f-9b0e-13a3a9e97384 for your component, change that to your own unique GUID.

BTW, you can easily get the page domain in jsPrintSetup.js with this command...
this.DOMAIN = this.getWindow().content.document.domain;
Then you just need some rules to compare it against (I just used a hard coded string comparison for my purpose).

This review is for a previous version of the add-on (0.8.2h).