Do you see an X in that windows upper right?

In order to better assist you with your issue please provide us with a screenshot. If you need help to create a screenshot, please see How do I create a screenshot of my problem?

Once you've done this, attach the saved screenshot file to your forum post by clicking the Browse... button below the Post your reply box. This will help us to visualize the problem.

Thank you!

What is in the sidebar?

You can check the home page setting to make sure you aren't opening a bookmark that opens in the sidebar.

• Options/Preferences -> General -> Startup: Home page
• https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

Firefox supports multiple home pages separated by '|' (pipe) symbols.

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