Do you mean the Bookmarks Sidebar opens on the left side of the tab? You can close that by pressing Ctrl+b, and Firefox should remember that.

Sometimes the file that stores the last state of Windows and toolbars becomes corrupted and fails to update properly. You can rename the file and customize from scratch.

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then rename localstore.rdf to something like localstore.old

Start Firefox back up again. Hopefully no bookmarks sidebar!

None of those took me to " Show Folder" ...

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

• Use a compressed image type like PNG or JPG to save the screenshot
• Make sure that you do not exceed the maximum size of 1 MB