GM_setValue get less able to setting big values(> 4kb).
Currently, max pref size is MAX_PREF_LENGTH = 1 * 1024 * 1024
I don't see any reason to make a change here, if a userscript goes over the new limit, then they there will be an error message. That seems fine to me. Taking the action that Greasemonkey did to create a separate storage seems too drastic to me atm. I'd rather continue using about:config prefs that can easily sync'd