Rated 4 out of 5 stars

I'd like to be able to open new tabs in the background by default. But can't do it. Or at least can't figure out how to. o/w love the app.

This review is for a previous version of the add-on (0.9.5). 

This can be configured in the Speed Dial settings panel ("Controls" tab); you can choose between current tab, new tab, new tab in background (obviously your choice), or new window. You probably want to modify the "Mouse click action" > "Regular" > "Middle click".