Theme Font & Size Changer lets you change the global font size and font family used for the user interface in Firefox. Design your browser with your favorite font.
Here is a little more about the add-on from the developers:
Theme Font & Size Changer is a simple browser tool that lets you change the font size and font family of Firefox. It is especially valuable tool for visually impaired people and wide screen users. Different from add-ons that enlarge and reduce text on web pages, Theme Font & Size Changer changes the font size and font family in all windows, menus, toolbars, etc within Firefox itself. All font size and font family changes are instantly applied and they are saved permanently and through sessions. You may assign your favorite font into your browser interface and change its size as you wish.
Interested in learning more? You can pick up the Theme Font & Size Changer from the Firefox Add-ons website.