10.10. My own signatures are invalid. Enigmail replaces “>” with “|” and spaces with “~” in quoted messages.

This is not a bug, but a workaround that Enigmail uses to handle the text=flowed format (RFC 2646), which causes problems with OpenPGP signatures.

Do not use this format in your mailclient; please set the option mailnews.send_plaintext_flowed to false. In Thunderbird, this option is under Tools → Preferences → Advanced → General → Config Editor. In SeaMonkey, this option is found by opening the URL about:config.