networking - IPv6 connection fails on Chrome, timeouts on Firefox

I noticed there's a certain page taking a long time (5 to 10s) to load on Firefox. I traced the delay and it happens when trying to connect to a certain host, bn.uol.com.br.

Strangely, this delay only happens on Firefox, but not on Chrome / Chromium. It happens on Firefox 31.0 on Ubuntu 14.04, happens on Firefox 42.0 on Windows 10, and happens on Firefox 42.0.1 on Android 4.4.4; but it doesn't happen on Chromium 45.0.2454.101 for Ubuntu 14.04, or Chrome 46.0.2490.86m for Windows 10 or Chrome 34.0.1847.114 for Android 4.4.4.

I want to end this delay on all OS'es and all devices, either by properly enabling IPv6, or by entirely disabling it.

I had previously noticed intermittent apt-get stalling on random IPv6 addresses. I suspect (but I'm not certain) my ISP doesn't enable IPv6, and I also suspect (but also not sure) IPv6 is disabled on my wireless AP/router. I got suspicious and performed the http://test-ipv6.com/ test on both browsers on all OS's (same wireless network, same router / AP).

Here's the results:

Firefox on Ubuntu

Chromium on Ubuntu

Firefox on Android

Chrome on Android

Firefox on Win10

Chrome on Win10

---

wget (Win 10)

There's a long pause before the IPv6 timeout.

More testes on a vanilla live USB Ubuntu

$ wget bn.uol.com.br
--2015-11-30 22:11:29--  http://bn.uol.com.br/
Resolving bn.uol.com.br (bn.uol.com.br)... 200.147.35.201, 2804:49c:319:430::126
Connecting to bn.uol.com.br (bn.uol.com.br)|200.147.35.201|:80... failed: Connection refused.
Connecting to bn.uol.com.br (bn.uol.com.br)|2804:49c:319:430::126|:80... [5s PAUSE HERE] failed: No route to host.

There's a long pause before the above IPv6 timeout.

$ ping6 bn.uol.com.br
PING bn.uol.com.br(2804:49c:319:430::126) 56 data bytes
From fe80::3e77:e6ff:XXXX:XXXX icmp_seq=1 Destination unreachable: Address unreachable
From fe80::3e77:e6ff:XXXX:XXXX icmp_seq=2 Destination unreachable: Address unreachable
From fe80::3e77:e6ff:XXXX:XXXX icmp_seq=3 Destination unreachable: Address unreachable
^C
--- bn.uol.com.br ping statistics ---
6 packets transmitted, 0 received, +3 errors, 100% packet loss, time 5009ms

$ ip -6 addr
1: lo:  mtu 65536 
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
3: wlan0:  mtu 1500 qlen 1000
    inet6 fe80::3e77:e6ff:XXXX:XXXX/64 scope link 
       valid_lft forever preferred_lft forever

$ ip -6 route
fe80::/64 dev wlan0  proto kernel  metric 256 
default dev wlan0  proto kernel  metric 256  expires 86397sec
default via fe80::9e97:26ff:XXXX:XXXX dev wlan0  proto ra  metric 1024  expires 297sec

The third line seems to point to my wifi ap/router, although I'd suppose IPv6 is disabled on it (it's a Technicolor TD5130v2 and the user interface is quite confusing)