Does exactly what I want - moving and resizing windows to fit various scenarios, including my second monitor. My only minor complaints are that it'd be nice to be able to set custom keyboard shortcuts for the various presets (though this may be a Firefox limitation, so no negative stars for this at all), and it'd be nice if it would detect if it's on a Mac and display the Option key instead of Alt in the text.

It's been a while since pop-up windows don't fit perfectly with the side edges of the desktop. despite trying to increase or decrease the distance from the left, nothing changes. for the left window I tried to pot a negative value but still the window is far from the edge

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....

Resizes windows exactly how and where I like them. :D

Great add on. :)