Firefox is not running in full window in tiny core linux

[whizzywhirl]

hey guys, so here are the things that I am doing
I am trying to run firefox at the start with no wm or any other app so my .xsession file as shown in the picture looks like this. Also, I removed wbar.tcz, Xprogs.tcz and flwm_topside.tcz from onboot.lst as I only want to run firefox. Also, I have a script in /home/tc/.X.d/ that runs firefox on boot.lst.
There are two major problems that I am facing as Firefox runs in full screen (as given in the script using xdotool key F11) the firefox is not in totall full window i.e it doesn't cover the whole screen. I have attached photo below in which the right side and the bottom side is not in full screen.
-> The second problem is I am trying this fix as given in this url https://support.mozilla.org/en-US/questions/1092572 . But,in the directory /usr/loca/firefox-esr/defauls/ I am unable to find the file userChrome.css and hence to apply the fix.