Rated 4 out of 5 stars

I just thought I should add this in here.

I looked for hours for an HTML source editor that would work with 3.1, to no avail. Then I stubled upon this:

https://addons.mozilla.org/en-us/thunderbird/addon/stationery/

It's not really built just for that, but it has a really nice HTML 'View Source' style editor in it. Worth looking at if you need that.