In my case Firefox starts with the side panel resuming in whatever state it was in when Firefox last closed. Have you tried closing all but one Firefox window, then ensuring that the side panel is closed in that last window, before closing that too? Hopefully Firefox will then start with no side panel from now on.

Hi TechHorse! Yes, I have tried closing the panel on the very last window and quitting Firefox. Also I've tried closing the panel, closing the last window, and then closing Firefox. The Bookmarks side panel was opened again the next time I start Firefox.

Keeping in mind this is on MacOS, it's possible to close all the Firefox windows without quitting Firefox. If I do this, the next window it opens will have the Bookmarks panel present even if I closed it on the last window I had closed. So it seems Firefox is acting like the last window closed has the Bookmarks panel displayed even if it was actually closed, regardless of how the last window gets closed.

You need to close all open windows except one window and use "Firefox -> Quit Firefox" to close Firefox. If that doesn't work, then remove xulstore.json in the Firefox profile folder with Firefox closed.

You can use the button in "Help -> More Troubleshooting Information" (about:support) to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thanks cor-el, deleting xulstore.json fixed it!