My current aDSL involves a Draytek DV120 in PPPoA to PPPoE translation mode, this connects to a Linksys WRT-54GL running OpenWrt 8.09.1 (Kamikaze). All was working OK, including QoS for my VOIP traffic, EXCEPT some sites just wouldn't load - Firefox (3.5) would just sit 'waiting' for the site to load. I did some googling around and found this page on the Waikato Linux Users Group site.
Once I added the following line to /etc/firewall.user on the WRT-54GL box all was sweet:
iptables -t mangle -I FORWARD 1 -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1444