extensions.torbutton.lastBrowserVersion, extensions.torbutton.updateNeeded and extensions.torbutton.lastUpdateCheck are not used anymore. We could also use this to double-check the existing extensions.torbutton* and remove the ones that are not used either. There's at least extensions.torbutton@torproject.org.description and extensions.torbutton@torproject.org.getAddons.cache.enabled which does not seem to be useful anymore.