I used a translator to understand your comment, so apologies if I did not understand correctly.
Firefox usually will show a vertical scroll bar when the content is longer than the visible height. However, sites can suppress the scroll bar using a style rule (overflow: hidden). Normally they would only do this if also enlarging the visible area. However, they might not have tested their approach in Firefox and perhaps their rules are not compatible.
It is difficult to suggest a solution without access to the site. If there is a community of users of the site, perhaps another user has come up with a workaround or identified the problem in more detail?
If the nested document is in a frame, you often can pop that document out into its own tab using this method:
right-click the page > This Frame > Open Frame in New Tab
However, I know that is not as convenient as seeing both panels at the same time.
