Since Firefox hides the scrollbar when it'**** in use, some of the boxes in WME may contain more than the visible lines, but it's not immediate.
For instance, in the suggestion recap box, or in the Add Menu.

I'd suggest adding a scrolling shadow, showing a shadow on the top when there is more content on the top and on the bottom when there is more on the bottom.
It comes from a CSS guru, Lea Verou https://lea.verou.me/blog/2012/04/background-attachment-local/

I also added the horizontal version and generated a couple of CSS classes I use on the sites I develop: https://codepen.io/massic80/pen/jOQWxNm
This way, anyone would notice there is something hidden under the bottom of the box: a shadow on the bottom, with the classes I added above (which don't show anything if not needed), would help user experience a lot.

I attached some screenshots here
https://www.waze.com/discuss/t/house-number-not-displayed-in-place-update-request/381738/12?u=massic80