Migrate away from localStorage — is not safe against browsing data wipe

[kav2k]

As a report shows, the preferences can get reset. Presumably, this is a result of "Site Data" wipe in Firefox.

As per MDN docs:

Although this API [browser.storage.local] is similar to Window.localStorage it is recommended that you don't use Window.localStorage in the extension code to store extension-related data. Firefox will clear data stored by extensions using the localStorage API in various scenarios where users clear their browsing history and data for privacy reasons, while data saved using the storage.local API will be correctly persisted in these scenarios.

A code performing a migration is needed.
Should also clean the mess from botched 1.5.1 upgrade (though it's harmless) and add storage versioning.