I can't select 10 px font sizes with the slider on some fonts like "Space Mono." I can with others, and then can click the "Apply to all specimens" to update ev

I can't select 10 px font sizes with the slider on some fonts like "Space Mono." I can with others, and then can click the "Apply to all specimens" to update even the fonts I can't adjust with the slider control. I'm using Firefox 47.0.