After setting up internet in our new flat, I wondered why there often was a delay of almost half a minute between entering a web address and firefox starting to load the data. I found out that firefox sent IPv6 DNS queries (type AAAA), which our Alice-DSL router didn't understand. After 4 tries had timed out, it finally sent an IPv4 query (type A) that succeeded. The solution with the best cost-benefit ratio seemed to be setting
network.dns.disableIPv6 true
in firefox's about:config. Strangely, though, I found an address of
an Alice DNS server (213.191.74.12 or dnsp03.hansenet.de) that didn't
show these problems when setting it manually in /etc/resolv.conf