My new laptop has a 17″ full HD screen and I noticed that in Firefox websites would appear larger than they’re actually are. At first I thought I had accidentally zoomed in a bit but Firefox reported it was at 100%. The cause of this behavior is because Firefox looks at the pixel density of your screen and bases its optimal default zoom level on that.
If you want 100% zoom to be really 100% zoom just browse to about:config and search for layout.css.devPixelsPerPx and change its value to 1.0.
Ah, changing layout.css.devPixelsPerPx to 1.0 in Firefox seemed to do the trick.
— Christiaan Kras ☮ (@Htbaa) September 27, 2013