- Determine profile folder:
Help > More troubleshooting information > Profile Folder
- Create a folder called chrome in the profile directory
- Create a file userChrome.css in the chrome directory with the following contents:
#TabsToolbar { visibility: collapse !important; }
- Navigate to about:config
- Search for
toolkit.legacyUserProfileCustomizations.stylesheets
and toggle it to true if false - Restart browser