When you double click on the empty part of the tab bar in Firefox (as well as IE, Safari & Opera) a new tab is opened. Now that Thunderbird has a tabbed interface it would seem to make good sense to follow a similar behavior. Given the options of what could be contained in the new tab, a new email composition would seem to make the most sense.

Well, given that we can't open compose in a tab yet (and doing so will involve rather a lot of refactoring of the compose backend), I hope you're not in a hurry ;)

If a tab-based version of the compose window is not planned for 3.0, I still feel that double clicking on the empty area should do something useful. Is there indeed no plan to have tabbed compose in 3.0?

If there is no plan for compose being tab aware then I think is useless to enable the tab feature at all for 3.0. Having a half implemented tab interface is more confusing than helpful.

Yep. Not opening the bookmarks manager and download manager in tabs is probably why Firefox has been such a miserable failure, we wouldn't want to go down that road.

No, but opening some pages in tabs and others in Windows would have. That would be a proper analogy. ;)