I believe I found the reason of this odious bug.
If, in firefox, it is set a minimum font size, as the I usually set, the bug is simple to view.
If the minimum font size is set to none, the bug disappear.
I do attach 3 screen shot I made to explain where I changed the minimum font size in FF to let you get evidence of the bug too.
In spite of this, I still believe that my suggestion of 21/Jan/11 11:29 PM here above is useful.
Hope you can now experience the problem.
Let me know, please.