External Editor Mod
When writing larger emails I love to use the External Editor Plugin for Thunderbird so that I can use gvim. One good practice is to set mail composing to plain text format. This avoids the problem of reconverting to a non-monospace font after closing the editor.
However, I experienced a harder-to-solve problem which is discussed in the forum as well: after closing the external editor quoted message parts have not been detected due to re-arrangements of the text. A simple fix is to call the menu command Edit/Rewrap. However, when altering the source code of the plugin a more ellegant way solves the problem without rearranging the message text. You may download the a modified version 0.7.3 from here. You just have to replace the corresponding file.