I assume the parent means native support for vertical tabs in the browser as opposed to through a third party extension.
Re:Mozilla’s Uncertain Future
by KingOfCoders on Saturday August 15, 2020 @09:54AM
(#24168865)
a.) Yes
b.) With changing their extension API several times it was not clear the plugin would make it.
https://addons.mozilla.org/fr/firefox/addon/tree-style-tab/
?