I don't like big buttons if they are not strictly needed. Rated 3 out of 5 stars

If you are like me who don't like extensions adding big buttons in your UI, you can look at "SmtpSwitch" extension (https://addons.mozilla.org/thunderbird/addon/4972). This one works well, though.

This review is for a previous version of the add-on (1.0.12).