webadmin: optimizing view for FF17

The "This Browser version isn't optimal" will not be displayed for
Firefox 17 on Linux, as Firefox 17 is an ESR version:
http://www.mozilla.org/en-US/firefox/organizations/faq/

Change-Id: I4fd5278f2a5778222f9e8f4d8b68dc4b5a144b9f
Bug-Url: https://bugzilla.redhat.com/891877
Signed-off-by: Einav Cohen <ecohen@redhat.com>