I was a fan of Tab Groups, used it all the time and got desperate when I heard it wouldn't be running on FF 57. I was considering writing an extension to replace it, but Fonso did all the work for me and that's wonderful! Of course I liked the full-screen display of all my groups and tabs in Tab Groups, but understand it's impossible to replicate. On the other hand, closing the unused groups or workspaces frees RAM, along with the new efficiency of FF 57. With FF 55 and Tab Groups I usually climbed up to 4GB (25% of my RAM), and at that point, FF began to slow down and bug; I knew I had to close it down and restart it. With FF57 and Tab Workspaces, FF uses only a few percents of my RAM! It's very useful because I multi-task a lot with memory-hungry programs (e.g., GIS software + Illustrator + InDesign + Acrobat etc).

Only 4 stars because I find annoying that switching to another workspace loads all the pages in this workspace, which (1) takes some time, (2) forces to go to some tabs to stop videos from starting. I would suggest :
-either to provide a list of the tabs when selecting a workspace, so you could choose the tab you want to open (but still display all the tabs, ready to be re-opened)
-or arbitrarily only open the leftmost or the rightmost tab for example (while displaying all the tabs as above)
Is it technically possible?