Tor Browser 5.0.4 has its SOCKSPort configured with "IPv6Traffic PreferIPv6".
This means that if your DNS resolver returns IPv6 records, Tor Browser and your Exit should default to using IPv6 to websites that have both an IPv4 and IPv6 address.
Is your DNS resolver correctly returning AAAA records along with A records for all sites that have them?
(If it's only returning AAAA records for a few sites, or for IPv6-only sites, that could be your problem.)
As far as I know, nothing changed in Tor Browser that could cause a dramatic IPv6 traffic drop-off.
Tim
Tim Wilson-Brown (teor)
teor2345 at gmail dot com PGP 968F094B
teor at blah dot im OTR CAD08081 9755866D 89E2A06F E3558B7F B5A9D14F