layout.css.devPixelsPerPx setting

Current version or firefox is Version 128.10.0esr

I had the same problem. After varying my windows display settings around, I found out that I had to fix it in the Windows display settings. What I did was:

     be sure that my laptop display settings are correct before connecting the monitor,
     connect the monitor and  Detect  it if necessary, 
     select  Duplicate These Displays  in the  Multiple displays   drop down list,
     identify the laptop display as number 1 and the external monitor as number 2, 
     select  show only on 2  from the list,
     check that the external monitor is shown as a shaded and number 2,
     set the the resolution and scale on  number 2 as wanted,
     leave the  Multiple displays  set to  Show only on 2.
   

Each time the monitor is connected or disconnected the display should automatically swap screens. To change settings on one of the screens that display must be the Show only on selection.

My laptop is almost always connected to the monitor, so I don't have change any thing when I use it. If I disconnect from the monitor my laptop picks up the display without any input from me. When I reconnect the monitor it picks up the display.