This sounds like https://bugzilla.mozilla.org/show_bug.cgi?id=679090 which is fixed and in the release schedule (looks like it should be fixed in Firefox 8)

I found the Firefox guide for problem solving: IPv6 can be disabled - and it was. Maybe Mint-11 has set that before letting Firefox loose, I don't know. I searched for IPv6, it gives "sites cannot be accessed which other programs can acces", - and there it was, you can disable IPv6, so I guessed that IPv6 was disabled, and I wanted it back.

IPv6

Firefox supports IPv6 by default, which may cause connection problems on certain systems. To disable IPv6 in Firefox:

  1.      In the Location bar, type about:config and press EnterReturn.
         * The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!, to continue to the about:config page. 
  2. In the Filter field, type network.dns.disableIPv6.
  3. In the list of preferences, double-click network.dns.disableIPv6 to set its value to true if you want to disable IPv6.

... or set it to false if you want IPv6 back!

I enabled it (default) and then Firefox responds to requests for ipv6.google.com and all the other sites.