It's very useful add-on for me. The only flaw is I cannot 1600 x 1045 for my ultra-wide LG displayer (resolution 2560 x 1080), I have to add 1600 x 1040 instead of. It's appreciated to have 1060 x 1045 in the updated version :)
You should be able to do this. In case you have multiple displays, be sure your FF window is on that particular one while the settings page is opened/reloaded for it to recognize the size of that display.
(There was a change in FF that broke the auto-updating of this value)
