Been using zen browser for quite some time. I use a session saver add-on and that one works weirdly due to how Zen has spaces and how that works. The way I understand zen's spaces is like they are browser sessions that can be run simultaneously. Each of them have their own pinned tabs for example, can have default containers they open in, on top of the universal whole browser pinned tabs they call essentials tabs; that's limited to 12 for now.

Sideberry and tree style tabs might not work as you expect because of the changes the zen team made