Have you seen Mozilla's "test pilot" collection of addons? They're experimenting with adding vertical tabs into Firefox: https://testpilot.firefox.com/experimen

Have you seen Mozilla's "test pilot" collection of addons? They're experimenting with adding vertical tabs into Firefox: https://testpilot.firefox.com/experiments/tab-center/ https://testpilot.firefox.com/experiments/tab-center/ It seems Mozilla is very much aware of what will disappear when WebExtensions becomes the only extension API, and what will need to happen to replace it.