Firefox and Opera denied access to directories

Hi All, I have a strange problem with firefox and opera. Neither program seems to be able to access files and directories which I have full access rights to, including at least some of their own config files. This covers opening local files, changing preferences, uploading files etc... This only happens on one machine. Can anyone suggest where I might look for pointers? I've checked permissions etc and they are all correct. Cheers Dylan -- "The man who strikes first admits that his ideas have given out." (Chinese Proverb)