I don't know what all the characters are in that text I just sent, inIt was probably a non-breaking space, originally.
You have Thunderbird set to encode your outbound messages in the Windows-1252 character set. One cure for that would be to set it to UTF-8, which will work better with most web sites and email systems (but work poorly with some legacy email systems, such as my Eudora Classic).
When seen by a system expecting UTF-8 that turned into the unicode Replacement Character - a white question mark inside a black diamond - a character used to replace an invalid UTF-8 sequence. That's what shows in the View Source view of your message in the Archives.
Then, that replacement character, seen in a system expecting Windows-1252 or one of the ISO character sets, would look like three symbols from the top of the character set.
The odder thing is that the text of your recent two messages are invisible in the Archive, except in View Source. This may have something to do with leaving quoted material at the top of the message.