Fix performance issue with MutationObserver in hideAmericanAndCanadianStatesFromFilter

[ineffyble]

From the FireFox add-on review team:

Here is another tip: use observer.disconnect(); before removing nodes in function hideAmericanAndCanadianStatesFromFilter (and if needed, use .observe(...) to continue listening again when the function finishes). Currently it is not a huge problem because you are only removing nodes, but usually it will lead to performance issues when a DOM mutation triggers the MutationObserver, which in turn triggers a DOM mutation, etc.