Hi again and sorry for my late reply!
I first had to create a test case that causes vertical overflow. For everyone reading this, here it is:
https://jsfiddle.net/SebastianZ/1gs0wx8t/
And yes, I can now reproduce the issue using Firefox 86.
Luckily, I can say that I obviously coincidentally already fixed this issue together with the changes in bug 1693147 that changes the display of the request body to use a toggle button to switch between raw and formatted view. Those changes are currently in Firefox Nightly and will ship in Firefox 88.
@snpro Could you please download Firefox Nightly and check if the problem is gone for you, too?
Sebastian