Prepare e10s (Firefox tabs in separate processes) compatibility
compatibility fix for Firefox < 17
interface "nsIMessageListenerManager" was called "nsIChromeFrameMessageManager" back then
Mark add-on multiprocessCompatible
register style sheets only once (instead of whenever loading a new document)
Make add-on restartless
Get rid of overrides
- Apply a custom class name to all media pages interactively.
- Load a single custom CSS file (via style sheet service) with all necessary customizations.

Advantages of this approach:
- More flexible.
- Doesn't require specific CSS for different Firefox versions (since only specific rules need to be applied instead of overriding the whole CSS file).
- The overrides weren't working until a restart anyway (rendering restartless functionality partially useless).
minor code cleanup