Which can be worth it depending on what you trade it for. I really liked the tab group feature but the new extension API is vastly more secure and can (probably) be extended to allow for most addons.
At Mozzila they need to prioritize many different things and I can understand how a constant push to make the browser leaner is foundamental. (especially in cases like RSS feeds)
It's true that it wasn't widely used, but it seems as if Mozilla did their best to make it that way. It wasn't exposed to the user by default, but instead was hidden as something you could get to by the toolbar customization screen, so you had to modify the toolbar to even get access to the feature.
Wouldn't that indicate that the extension was not that widely used, and as such Mozilla were correct in removing it from the central codebase?