Okay: the developer does spell it out — when you switch workspaces, the tabs of the previous workspace are CLOSED, not just hidden as they were with the (wonderul, much lamented) Tab Groups. It seems this is unavoidable with the new FF. But it guts the extension:

• When you switch workspaces, you have to wait for all the tabs to reload, and
• When you quit FireFox, only the tabs in the active workspace are saved.

I was using a different container (i.e. identity) in each workspace, with at least six tabs for each of four workspaces. That let me separate business, personal, hobby and second-business work — but having to set it all up again on restart is a bummer. If there's a way to save the workspaces and revive them, I'll up this to 4 stars. But the speed hit from reloading tabs when you switch workspaces puts 5 stars out of reach. Looks like a very good try at doing something Mozilla has sadly made impossible.

My workaround for now is separate windows for each kind of container tab, plus Tree Style Tabs (may have that name wrong)