Ah, it is related to Firefox, and it does happen in Firefox with my altered CSS.

Now I feel like a complete prat :-D. (see - it's still the real me, Ironbear)

 

(That reminds me, I must uninstall Firefox and start again with it. I made some 'speed-up' alterations to it suggested on a thread here somewhere and it's never worked properly since. That's why I've reverted to IE)