Code changes regarding HiDPI support for high resolution displays have landed in Firefox 22 and later (bug #844604). You can modify the layout.css.devPixelsPerPx and increase or decrease the value in 0.1 or 0.05 steps to adjust the size of fonts and other elements in Firefox. *The layout.css.devPixelsPerPx pref is a String value parsed to a float and allows to fine tune the dimensions of all elements (user interface and web pages) more precisely (resolution 0.1 or 0.05). *Change the default value -1 to 1 to make it work like in previous Firefox versions (100%) Start with a value of 1 and adjust this value with 0.1 steps or 0.05 for finer adjustments. *Use values between 1.0 and about 0.5 to reduce elements in size (do NOT go all the way to 0.05!) *Use values greater than 1.0 to magnify and make elements larger (percentage divided by DPI, % / DPI). *http://kb.mozillazine.org/about:config If web pages needs to be adjusted after changing this pref then you can look at the Default FullZoom Level or NoSquint extension. *Default FullZoom Level: https://addons.mozilla.org/firefox/addon/default-fullzoom-level/ *NoSquint: https://addons.mozilla.org/firefox/addon/nosquint/ Use this extension to adjust the font size for the user interface: *Theme Font & Size Changer: https://addons.mozilla.org/firefox/addon/theme-font-size-changer/