Comment 89 for bug 411358

This Gecko code should use gconf_escape_key() on aScheme:
http://hg.mozilla.org/mozilla-central/annotate/545887140a1b/toolkit/system/gnome/nsGConfService.cpp#l239

If the error message is happening with GIO and the libgsettingsgconfbackend then something there must not be escaping the '+' correctly.