You've just added a new signature in Thunderbird, and when you send a new message, it has a bunch of red lines around the layout of the signature, like on a screenshot below?
That's okay! Thunderbird uses these red borders to show you where there are HTML tables and cells on which the signature is constructed. The Table does not have any pixel border size, so you would not see the construction of the Table. However, those are not part of the HTML and should not appear in the sent/received message.
This is normal; you would see this in any compose 'Write' message window because a table has been used. If you were to insert a table as part of the section where you would enter the body of the email, you would see the rows and cells of the Table. Otherwise, how else would you know where those cells and rows or the entire Table are because the table border has no pixel size. Therefore, the Table would appear invisible and useless.
Do not worry. The recipient would not see it in a received email because the Table has no size set for the Table's border. So compose and send the message, and your receivers won't see these lines.
Oh, and one thing, the signature separator refers to the -- you usually see above the signature. It is a delimiter, meaning the program would know where the body of the email content ends and where the signature starts. This allows some programs to remove content below the delimiter when replying; otherwise, there would be loads of signatures in a message that has been 'Replied' several times.
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