Firefox on Windows is now following display scaling options which may render text larger on high resolution displays. Some users may notice that web pages and the browser UI appear magnified in Firefox 22. The layout.css.devPixelsPerPx preference handles this and lets you fine tune the size of all fonts and other elements in the browser. Start with the default value of the preference, i.e. 1.0. To reduce the font size, use values between 1.0 and 0.5 (do NOT go too low!), to increase the size of fonts, use values greater than 1.0. To modify it do the following: # Type about:config into the browser's address bar and tap on the enter key. # Confirm that you will be careful if the warning appears. # Filter for layout.css.devPixelsPerPx. # Double-click the entry and change its value - to default 1.0 for starters Note that changes take effect immediately. You can right-click the preference and select Reset to set it to its default value again.