I didn't know the old Add-On so I can just comment on this one. I like the idea, but it still has some bugs that need improvement. As stated by someone else:
- clicking the icon does only recreate new tabs instead of supposed ones.
- When I save a Layout and check "Save Tabs" its doesn't save them actually. In the list it only says the name, the windows but it doesnt show amount of tabs (theres nothing saved)
- the scroll sync doesnt seem to work reliably..
- as ppl mentioned for the older version, having multiple tabs in the same window would be nice, but maybe not possible with new Firefox, so windows is fine with me, since I can just "show" and "hide" them anytime.
- would also appreciate being able to save multible tabs in the Tile Windows. Maybe it's supposed to be included in the "Save Tabs" feature which unfortunately doesnt seem to work.