Change default zoom level in Firefox

Trick source

Needed for my 3:4 monitor (portrait mode):
If you want to set the text size/pixel density in firefox different from that of your system's settings like it was handled in prior versions, enter about:config into the firefox address bar (confirm the info message in case it shows up) & search for the preference named layout.css.devPixelsPerPx. double-click it and change its value to 1.0 (or any other zoom factor that fits your purpose; -1.0 is the default value and will adhere to the system settings). In my particular case I needed the browser zoomed at 90%, so the value put is: 0.9