I had this problem with Fx 1.x as well, and it is still present in Firefox 2.0.
When I load this address http://www.streetmap.co.uk/newmap.sr...2&y=219138&z=3 in Firefox, it is a matter of luck how many of the 9 map squares get displayed, usually zero, one or two. However in IE6 it always loads perfectly.
Any ideas, please, for anything I can do? Do other people see the same problem?