[asjo]

Not everybody is as computer savvy as you are - my parents don't notice tabs nor do they care about them.

So after a while their Thunderbird has hundreds of tabs open, without them noticing.

For them, it would be an improvement if tabs could be turned off (basically simplifying the UI).

[seorphates]

I don't understand how that changes anything about customization.

But I do understand the tab stance. When anything changes in UIs that directly affects the entire experience it is almost always a hurdle, for sure, no matter how savvy, no matter how willing.