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Updated for the latest (almost released) Thunderbird version, this is one of those necessary add-ons. You will wonder why this is not built-in in TB.
I have to drop a star for this last version though, because the new icons do a poor job of differentiating between in and out, and add to the "envelope inflation" that developers have been trying to reduce. Luckily, following the instructions on the ShowInOut website, you can easily change them, switching back to the old images.