Inconsistent Firefox screen resizing at session restore

[Daniel LaSalle]

Prelude:
I've have had this problem for MANY LTSes on both a Dell XPS13 laptop and tower and just now I can properly articulate it!1!

Anyways see attached picture for those thousand words I cannot express.

Context:
So after my session relaunches(RE: bug #1969815) so does FF (as it's part of my start-up apps) but even though it's contour/top bar seems to fully take the whole space of the desktop as it should that it's content doesn't agree.

Solution:
Clicking on the Restore button will actually drag back the contour around the currently amount of content and then re-clicking on the same button (Which will now be labelled "Maximize window") will do it's function properly.

PS:
- I think that I have been having this problem only with Wayland;
- No other software has given me such behavior.