That's annoying. Usually if the sidebar is open in the window you're viewing, and you launch a new window, the sidebar will be open in the new window, too. If the sidebar is closed in that window, and in all other windows, it would be hard to explain why it opens automatically.

On Mac, I believe the keyboard shortcut to open/close the Bookmarks Sidebar is Command+b. Pages can intercept that for their own purposes, so it may not work on some sites.

Thanks for the quick reply!

And you're right - it is annoying. Especially since I very seldom use the bookmarks sidebar, and the automatic opening of the sidebar just recently began.

Firefox provides various ways to close an open bookmarks sidebar, I'd just like to go back to where I was a week ago, when the bookmarks sidebar opened only when I pressed CMD-B, or pulled down the VIEW menu, or clicked on the bookmarks icon in the nav bar.

Try to rename/remove the xulstore.json file in the Firefox profile folder with Firefox closed.

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

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thanks!

It looks like removing a DuckDuckGo extension, making Google my default search engine, and restarting Firefox may have solved the problem.