Only lets me enter 1 of 4 values. No way to enter own values.

Why on earth does this extension not let you customize the font size? It's the only extension on firefox that lets you change font sizes, but for some unknowable reason the developer decided to limit you to one of four arbitrary font sizes instead of putting a text box. The screenshot shows the 18px, 20px, 22px, and 24px buttons, but it doesn't show that those are THE ONLY SIZES YOU ARE ABLE TO USE!