Greetings. To find the source of the problem, I would suggest first to re-launch Firefox without addons, it can be done by opening the menu -> Help -> Restart with Add-Ons Disabled... If it doesn't affect the problem, please try to disable using the Fn key for function key input. This option must be located within BIOS settings, should be called something similar to "Function key behaviour". When you switch this option, you'll be able to activate function hotkeys without pressing the Fn key. Just in case if it's the Fn key to blame here. Also, do you have ASUS Keyboard Hotkeys software running? If so, please try to stop the software and try again. Feel free to include any more information that might be helpful.