changed screen width test to a greater than check

Thanks for this handy little extension!

This is a simple PR to change the criteria for HiDPI mode to a `>=` check.  When your HiDPI scaling is 1.5 and your screen width isn't evenly divided by 1.5 (e.g. 3200, 2560), firefox will toggle between the modes with every refresh.

If you also had heterogeneous displays, (e.g. a HiDPI laptop display, a 4k display, plus an older 72 DPI monitor), this would allow you to set a threshold at which to enable scaling.