We like this app. I like the Tree and Group functionality together. My wife keeps over 1000 tabs up. :)

One problem: On our unit, when you add more than a screen's worth of groups, no up or down arrows appear to allow you to move to groups that are past the screen's view. We use a laptop without a scroll mouse on a normal basis.

(Oh...one other thing. This may not be the add-on, but Firefox tries to make a new window whenever we try to drag and drop within the tree or to a group within the bottom inch or so of the browser. Our sidebar is on the left, and the browser is set to open new windows in tabs.)

Thank you for your efforts!