To force Firefox to use IPv4 DNS resolution instead of IPv6 go in your about:config tab and change the value network.dns.disableIPv6 to true

The little story: Last week-end I started the migration of my private network from IPv4 to IPv6. As I'm temporarily back at my parents place I do have to maintain a certain internet-availability.
An IPv4 to IPv6 migration is not that easy, especially when your ISP is still speaking IPv4. This means I have to tunnel all my IPv6 traffic to a tunneling service.

All this means quite some complex configurations as I want to do all the intelligence on the Linksys WRT54GL v1.1. This device is a little limited in memory and is giving me some issues with the DD-WRT firmware and the extra software I need for this. Well, it just takes more time than expected. So having a mixed (working) IPv4 and (not working) IPv6 internet gave me an unexpected problem: Firefox will use IPv6 name resolution if available. If your IPv6 packets don't reach further than your router, you just can't access all the sites anymore...that's why forcing IPv4 in Firefox can be useful.

sudo fs_usage -f filesys

Note to myself: Please don't forget to always put

ServerAliveInterval 60

This way your SSH tunnels will stay alive even if you're not using them.