Might you be closing Firefox with the Bookmarks Sidebar being open?

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

kiannaj said

Every time firefox launches on my PC the bookmarks sidebar is automatically open and i can't seem to stop this happening.

Hello kiannaj,

Just in case none of the above solves the problem; would you please take a look at this bug report and see if maybe what's mentioned there applies to your situation :

https://bugzilla.mozilla.org/show_bug.cgi?id=1498847

^: That is a really weird bug report where a user had copied the full content of the prefs.js to user.js with caused some migration code to run each time Firefox was started.

The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session. You can delete the user.js file if you have this file.

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