Rated 4 out of 5 stars

I rate this 4 stars - the ability to edit subject lines is very useful. I'd like to see this built into Thunderbird itself.

Invoking this is a bit clunky in TB3 - you have to look for "Change Header Details" in the Message pull-down menu from the main tool bar.

As mentioned in a prior review below, there is a simple hack to enable this for TB3. To get a copy of rev 0.7.1 with this hack go to:
http://forum.addonsmirror.net/index.php?showtopic=1906&st=0&gopid=38064&#entry38064