Not showing the correct resolution? Rated 4 out of 5 stars

Looks like a great add-on for working with responsive design... My laptop has a 1920px width and when the window is maximised it's showing 1936 x 1186? And it makes no difference if there's a scroll bar showing or not?

Thanks

I have explained what you're seeing in this reply to another comment:
https://addons.mozilla.org/en-US/firefox/addon/firesizer/reviews/512778/