The "fixed prefix" mean that the ISP provides each client with at least a /64 prefix, preferably a /56 or /48 (this is the equivalent of "fixed IP" for IPv4).
Most professional ISPs do this; those currently known to do this are:

1. Nerim (France)
2. FDN (France)
3. Free (France)
4. OVH (France)
5. Internode (Australia)
6. TekSavvy (Canada)
7. Andrews & Arnold (AAISP) (UK)
8. Tiera (Russia)
9. XS4ALL (the Netherlands)
10. OTE (Greece)
11. Forthnet (Greece)
12. [..] Please complete with ipv6 help pages from IPv6-friendly ISP

1. RCS-RDS (Romania) - offering both a /128 address and optionally a /64 PD via DHCPv6 (sla_len 0 in dhcp6c)
2. Comcast (USA) - DHCPv6: /128 for the immediate device (router) and a /64 prefix delegation. Requires wide-dhcpv6-client configured with "option 'na' '1'" in /etc/config/dhcp6c i.e. set Non-Temporary Address. Without changing the "na" option you will not get an address assigned to your WAN interface.
3. Time Warner Cable (USA) - Offers /64 prefix delegation with "option 'sla_len' '0'" and a /128 address for CPE with "option 'na' '1' when configured with wide-dhcpv6-client.
4. Telecom Italia (Italy) give a /64 dinamic subnet
5. Telekom Deutschland/DTAG with "IP-Anschluss" - new /64 prefix at every pppoe dial-in and then a RA with this /64 every 10min (to poll a RA use 'rdisc6 pppoe-wan')