penders - 9:05 am on Oct 21, 2013 (gmt 0)
When I typed chrome://browser-region/locale/region.properties in the address bar I saw which includes a whole lot of stuff:
This was just for reference. Mines a reasonably fresh install of FF 24 so maybe the exact output will vary.
The fact that this same URI is the value of mailto.1.name, mailto.1.uriTemplate, mailto.2.name and mailto.2.uriTemplate suggests it's probably just a developers reference. For me, that document is fully commented and identifies what each setting is.
When you say you changed mailto.2, I assume you changed it in the about:config page.
Did you then increment the version number in the chrome://browser-region/locale/region.properties to make it work?
Increment the version number in about:config. Search for gecko.handlerService and you should see all the settings. Restart FF and voila.