hello, developers took the decision to switch off hidpi support for dual-display systems again per default until an underlying issue is fixed in firefox 20... however you can manually enable it again. enter '''about:config''' into the firefox location bar (confirm the info message in case it shows up) & search for the preference named '''gfx.hidpi.enabled'''. double-click it and change its value to '''2'''. however there will then be the unadressed bug that menus might show up on the wrong monitor (until fixed later). https://bugzilla.mozilla.org/show_bug.cgi?id=832591