This looks more like a bug in Firefox to me - see
https://bugzilla.mozilla.org/show_bug.cgi?id=633001. I can reproduce the problem when using the raw (bracketed) IPv6 address, but it works fine when using a proper hostname (that maps to an IPv6 address only) instead of the raw address. Same problem with Safari under Windows. Google Chrome, Internet Explorer and Opera don't have any problems with the raw address though.