There’s no API to do that yet from a WebExtension, but they’re working on it: ht...

robin_reala on Sept 30, 2017 | parent | context | favorite | on: It’s time to give Firefox another chance

There’s no API to do that yet from a WebExtension, but they’re working on it: https://bugzilla.mozilla.org/show_bug.cgi?id=1332447 . In the meantime it’s possible to edit your userChrome.css manually to select the tab bar and hide it.