I did not change any default settings for FF 57. Just checked the config: layout.css.devPixelsPx, default, string, -1.0 Edit: Checked the screen resolution as

I did not change any default settings for FF 57. Just checked the config: layout.css.devPixelsPx, default, string, -1.0 Edit: Checked the screen resolution as well: it is default, but not scaled.