Earlier quoted context omitted.
They are trying to move away from the old extension API (because maintaining that ate up a crap ton of resources, so don't come up with that move being a waste of resources) and Tree-Style Tabs is not yet compatible with this new API. The Vertical Tabs test pilot thing was them demonstrating that it could be done (including to themselves, they need to test these APIs) and giving the community something to work with,…
>The Vertical Tabs test pilot thing was [...] to test these APIs Except that Tab Center wasn't a way for them to test the new WebExt APIs, as it was still based on XUL. See here: https://discourse.mozilla-community.org/t/whats-all-this-abo... >maintaining [the old extension API] ate up a crap ton of resources, so don't come up with that move being a waste of resources Arguably not a waste if that meant preserving the…
If the Firefox team wanted to improve speed/security/modularity, etc. they had to ditch the old extension model.
It sucks, but necessary.