Same flaw as this add-on's clone or original, WindowSizer. Only working half-way, at best!
No matter the actual window size, if whether maximized or not, this add-on always detects and displays a window size of 632x223, and offers to use that – and only that size for a new resolution to save. Of course, the result is an unusably minute letter box size browser window without any page content visible.

I suspect the add-on's inability to detect the correct window sizes stems from the browser's protection against fingerprinting. However, other similar add-ons don't have this problem with identical Firefox settings, so this is a no-go for me with this WindowSizer add-on.

Actually resizing a window might work, though, once one has cumbersomely and by hand set up a desired window size. I didn't stick around long enough to try.

Refreshing the window allowed to to return to the stupid full-screen. I need the SIZE & POSITION of the window to be fixed....