I installed Mozilla 1.6 yesterday and I now occasionally get the message "The file //forums/render/forum/show?forumid=<xx> cannot be found. Please check the location and try again." when I use the "Back" button to go back up a level. It happens maybe 1 in 10 times.
This is probably a Mozilla bug. I only mention it here because it has only happened on this site so far. I'm not sure how to report it to Mozilla either.
Hugh