Hi Aveslinger, I think this will be help to you

Open firefox click the SETTING (it's in the top-right corner of the browser) click the CUSTOMIZE button (it's at the bottom of the setting menu) click the SHOW/HIDE TOOLBARS (its near the bottom-left corner) click on the BOOKMARKS TOOLBAR (if the option is unchecked, that means the bookmark toolbar is already removed) click EXIT CUSTOMIZE (this will save your customization setting and remove the toolbar. you should see the checkmark disappear, indicating that the bookmarks toolbar is removed.)

Try to rename/remove xulstore.json 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 Folder/Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/Profiles

Thanks Dinushi Dhananjani, but I was only interested in removing the bookmarks sidebar.

Thanks cor-el. Your solution worked.

Thank you cor-el – that finally worked to remove the pesky sidebar.

However, I had to manually close down the sidebar on every open page before quitting Firefox and then remove xulstore.json (from each of the two folders under Profiles on the Mac)