Favourite Colour

Shows and stores your favourite colour, in storage.sync in the extension's about:addons page.

Demonstrates:

storing data with storage.sync
reading data from storage.managed
creating an options page and opening it with runtime.openOptionsPage()
best practice for supporting automatic theme switching (dark theme) in the options page

To have Firefox read data from storage.managed, create a file with this content:

{
  "name":  "favourite-colour-examples@mozilla.org",
  "description": "ignored",
  "type": "storage",
  "data": {
    "colour": "management thinks it should be blue!"
  }
}

Name the file "favourite-colour-examples@mozilla.org.json" and follow the instructions in the storage.managed documentation to make the file discoverable by the extension (on OS X and Linux this means copying it to a specific location, while on Windows it means creating a specific registry key pointing to the file's location).