Thunderbird can change the view mode of message and compose new message between HTML and Plan Text, but often that we set a default mode and need to view a message in another mode temporarily, also that thunderbird can reply a HTML mail by hold shift key when your setting is text, but thunderbird just set the new message as HTML but not load the original message as HTML.
Yes, these are what this add-on do! You can change the view mode temporarily without changing the default setting, and you can reply a message with full HTML of the original message.
Also that you can forward or print a message in HTML mode temporarily, but there is a bug in Thunderbird(https://bugzilla.mozilla.org/show_bug.cgi?id=228562
so that when your default setting is "compose message as Plan Text", foward will not work well, I just add this function and wait for the fix from Thunderbird...(fixed in 3.0)
Thunderbird 3.0 compatible new version 1.0.2 released, you can download from here or the homepage below.