As I am a corporate website administrator and I use an ultra-wide monitor with a resolution of 3840x1600 pixels, I need to be able to run firefox with window size of full HD to verify the compatibility of the site for common displays. '''How can I ensure that the Firefox window always starts at this size?''' In the case of Chrome, you just add the parameter -window-size=1920,1080 Unfortunately, this solution does not work here (so it worked in older versions). I found some plugins that promised this, but none worked correctly. I find the option to set a default size to be very basic and essential and prevents me from switching from Chrome to Firefox.