They made a pretty huge deal about the Tab Groups (aka Panorama) feature of Firefox 4 in 2010; designed by Aza Raskin! https://web.archive.org/web/20101105212333/http://www.azaras...
But then it was removed in 2016 because few people really used it (or few people who had telemetry enabled, at least): https://support.mozilla.org/en-US/kb/tab-groups-removal
Relying on telemetry is heavily biased towards people who do not know what telemetry is.
This whole anti-telemetry outrage is so manufactured and boring.
(1) A significant portion of Firefox users think like me. In this case, Mozilla should not rely on telemetry because it excludes a sizeable portion of their userbase, and one that likely correlates with things like "believes in Mozilla's stated core philosophy" and "is passionate enough about browsers to recommend them to people".
(2) Not many Firefox users think like me. In this case, it's pointless for me to enable telemetry because they'll just ignore my needs anyway. "It is not worth supporting this feature, the data shows that 99% of people do not use it!" won't change if the number is actually 98.99%.
More importantly, it seems both of them have survived the move to Quantum. It was really hard for me to imagine going without Panorama.
https://addons.mozilla.org/fr/firefox/addon/tree-style-tab/
?
b.) With changing their extension API several times it was not clear the plugin would make it.