HTTP 404 errors when using yum

When trying to install firefox using yum on a CentOS server I found that I was getting HTTP 404 errors when yum was trying the various mirrors. All these mirrors were in America.

I had fastestmirror installed which should pick up local mirrors which should be quicker. Many people have suggested altering yum configuration files to specify country codes but I found that yum clean all worked for me and pulled firefox from a UK mirror on the next attempt.