Yes, you can.can I set it in .xinitrc?
/usr/local/bin/firefox -geometry 1600x900+0+0 assuming that is the screen resolution being used. You need to adjust the geometry in your command accordingly. For example, if you have a panel at the top of the screen that is 20 pixels wide vertically and you want Firefox to open using all remaining desktop space, the command would then be /usr/local/bin/firefox -geometry 1600x880+0+20. See the X(7) man page for more information on geometry