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 632x119, 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.

The setting of the window size itself might work, though. I didn't really take the time to measure whether the resulting window really had the 632x119 pixels. ;)