Are you getting the green Keep Open page in any window? Usually at startup it appears in one window (sometimes twice for me; I restore my previous session, if that is relevant), and then if you launch a private window it should appear there. Note that in Firefox 67+, you now need to give each individual extension permission to run in private windows.
UPDATE: I added a toolbar button to version 0.6 so you can open the green Keep Open page manually if it doesn't open automatically.
