I'm sure that your suggestion to use PHP include is far better than frame but I just don't have the know how at the moment. I'll post it on the stuff to learn list (which is quite long).
Weirdly though, while I'm sure that you've pointed out the problem, today the page seems to work OK anyway. This is in line with what was happening before. Sometimes the iframe appeared in firebox other times not. I don't really get it, but I'm putting your advice into action just to be safe.