UriSchemes/chrome

In .mozilla/connolly/4evx0poc.slt/localstore.rdf I found:

<?xml version="1.0"?>
<RDF:RDF xmlns:NS2="http://home.netscape.com/WEB-rdf#"
         xmlns:NC="http://home.netscape.com/NC-rdf#"
         xmlns:RDF="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
  <RDF:Description about="chrome://messenger/content/SearchDialog.xul#priorityCol"
                   ordinal="19" />

What is chrome:? Where are the docs?

Now I remember a reason why not create new UriSchemes: you can't FollowYourNose to look them up.

Used by: internal to Mozilla; refers to user interface resources

Not in IANA's list