It seems like a problem with your operating system. Is it only on firefox? only on certain sites? only some programs?

It's only on Firefox and began about a year ago; there is an issue specific to Firefox. I've been using Firefox daily on Windows 7 Ultimate since the initial release. I've also been using this same monitor setup in a portrait orientation with Firefox since the Dell 30" monitors were released about 7 years ago.

If there is a checkbox, the closest I can click to select is the top right corner, but the center of the target to click is just barely upper right on the outside of the box. A radio button acts the same but I have to click outside the circle in the same upper right position since it's a smaller target.