It seems pretty decent, once one understands at least some basics. However, although I can't comment on long use data loss, which other users obviously experienced, since I am only using it for a few days now, for me, I have two main problems with this add-on:
(1) If Firefox is *not* open and I click on a link in another app which opens Firefox, then Firefox is opened *but* the link is not followed. I have to go back to the app and click the link again after FF is open. This sucks.
(2) The context menu of the "tabs" in the side bar to not offer any tab context menu items but *only* custom "Simple Tab Groups" items. This defeats the purpose of the side bar for me, which would be that I could turn off the main tab bar. But then I do not have a tab context menu anymore with this add-on. So, as a work-around I am using the "Tree Style Tab" add-on in the side bar and for now the "Simple Tab Groups" in the tool bar.