While doing work on the the Rooster Teeth Site Extender extension, I was having a serious issue with it not working. It kept logging a paticular error in the javascript console:
No chrome package registered for chrome://rtse/content/browser.xul .
It turns out that all package names must be in lowercase, otherwise the package just won’t work. So, when you are defining your package name, make sure you use all lowercase, otherwise you’ll get the same error.