Rated 4 out of 5 stars

I use this primarily to edit subject lines, and I find it very useful.

To use, look for "Change Header Details" in the Message pull-down menu from the main tool bar.

As mentioned in a previous review, there is a simple hack to enable for TB 3.0. You can get a copy of rev 0.7.1 with this hack here:
http://forum.addonsmirror.net/index.php?showtopic=1906&st=0&gopid=38064&#entry38064