Using opt-in telemetry might be a good way to assess the workflow of users in order to present a more streamlined UI.
Things that a user uses frequently is the groundstone (made up) of the toolbar paradigm.
Frequently used functions, when presented appropriately is equivalent to a powerful and intuitive interface.
I lost the previous draft of this message which was a little more expressive, but one should generally get the idea.
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If a theme includes a custom toolbar layout + extensions, and it becomes the most popular alternative theme, this would be a more accurate way to determine what the default should be. Telemetry is sub-optimal because many users do not know what features are available, so they never enable features which they might use. I have created some better designs, but theming is a nuisance on Thunderbird due to the lack of a theme editor.