I have a Google Chrome extension which does chrome.storage.local.set in
options.js and then gets it in a content script. The get is async so I
had to put most of the subsequent code into the callback function. If you
don't do that I guess the option might be empty.
I haven't implemented this in Fx yet - I want Android support - so I
don't know if Fx would be the same, but I expect so.