Otherwise, new extensions in the user, system and application extension
directories would silently be skipped:

~/.mozilla/extensions/{ec8030f7-c20a-464f-9b0e-13a3a9e97384}/
/usr/lib/mozilla/extensions/{ec8030f7-c20a-464f-9b0e-13a3a9e97384}/
/usr/share/mozilla/extensions/{ec8030f7-c20a-464f-9b0e-13a3a9e97384}/
/usr/lib/firefox/browser/extensions/

We set extensions.autoDisableScopes to 11 (1 + 2 + 8,
    PROFILE = 1, USER = 2, APPLICATION = 4, SYSTEM = 8, TEMPORARY = 16),
which means only extensions dropped into profile, user or system dirs
are disabled by default. (Mozilla defaults to 15 = 11 + 4)