XPI is the file extension used by Mozilla to package the extensions to Firefox and Thunderbird.
The XPI file itself is simply a "wrongly" named ZIP file containing specific information.
Ad blocker interference detected!
Wikia is a free-to-use site that makes money from advertising. We have a modified experience for viewers using ad blockers
Wikia is not accessible if you’ve made further modifications. Remove the custom ad blocker rule(s) and the page will load as expected.