Rated 5 out of 5 stars

I can see the effort is there, but none of the region controls displayed on screenshots are showing up for me. Window Master toolbar icons are there, but only display the settings menu.

I'm on Linux, Gnome 3.20 Wayland

This review is for a previous version of the add-on (7.3). 

Window Master should work fine with e10s, but it may fail on Linux if your screens are not configured as an extended desktop (although a control for a single screen should still be shown in that case). Has it worked on your system before e10s was switched on?

You may want to try the following: Open the Browser Console with Ctrl+Shift+J and then disable and re-enable Window Master in the Add-ons Manager. If Window Master works after that, it is probably an application start-up issue. In case it does not, you may want to check whether an error is logged in the Browser Console which refers to Window Master; if so, please report the error message back to me.