Position relative to current screen

[fuzzykiller]

A user reached out via e-mail to request positioning relative to the screen the Firefox window is currently on. Setting “Left: 50px” would then not move the window to the primary screen, if it is currently located elsewhere. Instead, it would be 50 pixels right of the current screen’s left edge.

Currently, coordinates are absolute, ie. relative to the multi-screen origin.

This enhancement would greatly improve multi-screen usability of the add-on.