Rated 1 out of 5 stars

it reset my DPI settings ( layout.css.devPixelsPerPx ) without asking and setting it back to the old value

Unfortunately, certain functions in Mozilla Firefox do not handle scaling correctly for multiple screens when layout.css.devPixelsPerPx is set manually. This user preference must be reset to enable Window Master to function correctly. Just uninstall Window Master and reset your user preference if you wish to use the flawed Firefox code.