This is an attempt at running Firefox in a NixOS VM. I've tried Playwright, but maybe a "real" Firefox will allow something more accurate (e.g. start Firefox with a configuration for high-dpi display).

Run the VM interactively

$ scripts/runvm.sh

Use Ctrl-Alt-F to exit the fullscreen mode, and use Ctrl-Alt-Q to quit QEMU (this doesn't work while in fullscreen).

Run the VM as a NixOS test

Disabling the sandbox is necessary to allow the VM to have network access. The result contains a screenshot of the Refli web site.

$ nix-build --no-out-link vm-test.nix --option sandbox false

Notes

The above runvm.sh script is already configured to do the "right things" but here is how I achieved it manually first.

In the VM, to use 2880x1920:

$ gtf 2880 1920 60
$ xrandr --newmode "2880x1920_60.00" 473.06 2880 3104 3424 3969 1920 1921 1924 1987 -HSync +Vsync
$ xrandr --addmode Virtual-2 "2880x1920_60.00"
$ xrandr --output Virtual-2 --mode "2880x1920_60.00"

The corresponding NixOS bits are

  services.xserver = {
    monitorSection = ''
      Modeline "2880x1920_60.00" 473.06 2880 3104 3424 3968 1920 1921 1924 1987 -HSync +Vsync
      Option "PreferredMode" "2880x1920_60.00"
    '';
  };

In Firefox about:config: layout.css.devPixelsPerPx is set to 0.9. Then restart Firefox.

The default font size is set to 14.

In the console, window.devicePixelRatio equals 2.4. (When layout.css.devPixelsPerPx equals -1, it equals 2.60869).

This means that on a fullscreen 2880x1920 window, Firefox reports a viewport of 1200x800.

noteed/firefox-vm

Run the VM interactively

Run the VM as a NixOS test

Notes