It's very sad this plugin was abandoned 5 years ago, it had a lot of potential. Specially nowadays as other browsers like Vivaldi and Arc come with very powerful multitasking tools.

If at some point, Firefox decides to give support to this plugin again, two suggestions I think would help a lot:
1. Be able to open the side view with a hotkey
2. Be able to navigate (through URLs) in the side view, instead of being stuck in the page you opened it