Rated 4 out of 5 stars
Does not function under Thunderbird 11. If you have problem with Thunderbird 11 (you can't access your tabs anymore) deactivate this extension.
Edit: even with 1.5.3, the problem is the same. I've tested playing with the "autohide" option, nothing changed.
In fact, the toolbar is now in the tabs, and if it's OK for the mail tab, the lighting tabs (agenda and tasks) are not showing the buttons. So the tabs other than the one displayed become inaccessible.
I assume you used the feature hiding the tabbar?
There was a issue, which is now fixed in 1.5.3. You can download it at https://addons.mozilla.org/thunderbird/addon/lightningbutton/versions/1.5.3
EDIT: Note that the no-tabs feature is NOT ment to be used without the Add-on Rise of the Tools or putting the buttons to a always-visible place (menubar).