I post this in the hope that it will save someone the headaches I suffered:

I found that certain web sites were unusable (super slow – minutes to load) in firefox on linux, although retrieving pages from the command line w/say wget worked just fine – python.org, for example, and the firefox addons site. It turns out that DNS records for these sites include ipv6 addresses, and I guess we can’t route ipv6 packets properly? I was able to fix the problem by disabling ipv6 on my box:

in /etc/modprobe.d/blacklist I added
blacklist ipv6