Theme Font & Size Changer is a free browser tool designed for Firefox that allows users to customize the font size and family across the browser's interface. This tool is particularly beneficial for individuals with visual impairments or those using wide screens, enhancing accessibility and usability. Unlike typical text enlargement add-ons, it modifies fonts in all windows, menus, and toolbars, ensuring a consistent experience throughout the browser. Users can quickly adjust settings via a toolbar button or the Tools menu, with changes applied instantly and saved across sessions.

The program supports a variety of font adjustments, including size, family, style, weight, and color, allowing for a personalized browsing experience. Additionally, it functions with other Mozilla products like Thunderbird and SeaMonkey, providing similar customization options for their user interfaces. While primarily designed for the default Firefox theme, it may also work with non-default themes, though some elements may not respond to the changes. Overall, Theme Font & Size Changer offers a straightforward solution for optimizing text display in Firefox.