Awesome addon! FireFox ver. 57 came out only a month after I discovered the incompatible Session Manager addon, and it was disappointing to stop using it right after finding out how useful it is. This addon is a WIP Session Manager replacement for FF 57, and so-far it works great. The best part is that I can directly import the sessions from SM I had saved, which is super convenient. Other people seem to be having some problems and it doesn't have all the features that SM had, but it works fine for me.