userChrome.css to hide Firefox native tab bar and extension sidebar header (for use with TreeStyleTabs)

userChrome.css

/*
 * about:config -> toolkit.legacyUserProfileCustomizations.stylesheets : True
 * about:support -> Profile Folder -> Open Folder
 * Create `chrome` direcory
 */

#tabbrowser-tabs { 
  visibility: collapse !important;
}

#sidebar-header {
  visibility: collapse !important;
}