Rated 4 out of 5 stars

Good add-on filling the gap of Firefox missing "Move Tabs to Existing Window" functionality. Since it equally targets single and multi-monitor systems, would be great to show existing windows clearly recognizable for easier target window selection.

Open windows can be presented in Context Menu under currently active in each window Tab names, or shown as small popup one tab pictures similar to Windows Taskbar icons behavior on hover. But they should be easily recognizable anyway. IMHO, its the major drawback of this add-on for now. Hopefully, will be fixed in the next update. But its approach of easily separating tabs between different monitor screens is unique.

Also, on FF toolbar the WM icon dropdown options better contain intuitive icons, with Tooltips for each option shown on mouse hover. As well, when "Move Tab to a new Window (free space)" is chosen, an option for Cascading the new window with existing ones would help, instead of placing it in-line, even for a single small monitor with little unoccupied screen space, so the new window is created very narrow.

This review is for a previous version of the add-on (7.0). 

You might want to hire me for a few days/weeks to enable me to implement your suggestions.