startup: Lower blacklist for Firefox from 4 to 3.

Though Firefox is 3.6 is both unsupported by Mozilla, by jQuery
and by MediaWiki JS, it is not known to have fatal errors in
javascript run time execution so moving from Grade B (blacklist)
to Grade X (unsupported but given Grade A source code at own risk).

This is done following a thread in English Wikipedia's Village
pump requesting to get the Grade A resources again.

Follows-up 02fe025e6364ebf6f.

Change-Id: Ia54dd738b3ce0995fab6aae7a2729cc7b1a0dcb1