ViewSourceWith (VSW for friends) is definitively a browser oriented extension, using it with Thunderbird (TB) produces a very different experience.
VSW under TB allows users to view the mail message text (for example to inspect headers) but it doesn’t support any editing feature.
Under Firefox VSW offers textbox form editing (single and multiple lines) and I use daily the feature to edit GMail message content.
Editing rich textbox like GMail is a great feature, you can edit HTML code not only plain text, this is a great step ahead.
A couple of users asked me to add to VSW the ability to edit Thunderbird messages, not the raw view but a real edit feature in mail composition.
Well, now VSW (from version 0.3) can be used to edit Thunderbird messages, both plain text and HTML messages.
Technically speaking adding TB editing was enough easy using the nsIEditor, the same component used under Firefox with GMail.
I hope to post more details about the nsIEditor usage in VSW