When using Firefox version 22 and you add (twitter.com) to blacklist in SWG UI, the site does not get blocked.
Firefox redirects the twitter.com site to an HTTPS site. When SWG is in Inline Blocking mode only the SWG cannot inspect HTTPS traffic therefore the site will not get blocked. SWG is working as design. HTTPS traffic can only be inspected when in Inline+Proxy blocking mode or Proxy blocking mode and SSL Deep Inspection is enable. See Solution for workaround.