I have been using this extension for several days now, closed and re-opened FF (Developer's Edition) several times. It works perfectly and it is the best replacement I found for Tab Groups so far.
A great point: only the active tab loads in a given group, which saves RAM. So, as I used to do with Tab Groups, when FF takes too much RAM, I just close the program and re-open it.
Small things to improve:
-tabs in each group are not always shown in their actual order
-it would be nice to be able to give a group a specific name