Hi Peter,
This is a known problem when viewing WebHelp in Firefox. The
only solution I've found is to set the popup size manually, which
is a bit of a pain if you have a lot of popups! It also means that
if you change the amount of text in the popup topic, you need to
resize the popup again. Not ideal, but maybe it will be fixed in
the next release of RoboHelp. (We live in hope.)
Anne