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Works exactly as expected for HTML composing. Very clean interface. My only (minor) regret is that it occupies an extra row of screen space.
I wish I knew what it is doing when composing in plain text. The HTML tab is still there, showing HTML with a body styled in monospaced font. Is it actually sending HTML when I'm asking for plain text? Is this just an artifact of how Thunderbird produces a plain text email?

Tato recenze je pro předchozí verzi doplňku (2.0.0). 

So-called "plain text" is also HTML behind the scenes. The HTML tab shows you what's going on. That will allow advanced users to "fix" quotes, etc. IOW, when you ask for plain text, plain text is sent, but during composition, the editor is manipulating HTML.