Can you not close the sidebar? There should be a close button in there.

Of course there is a close button, the shorcut ctrl+H works too. The problem is I don't want to do it everytime I open a tab (about 1k times a day).

Thank you

If you close the sidebar and then close the application, it should open again with no sidebar. If that is not the case, maybe the xulstore.json file in the profile is corrupted.

Try to rename (or delete) the xulstore.json file in the Firefox profile folder.

You can use this button to go to the currently used Firefox profile folder:

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

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes