Markdown Here appears to be unmaintained and is certainly incompatible with Thunderbird 68.0. I have not been able to raise the developer of it at all and PRs are not being merged.
The idea behind Markdown Here is good, but it has some issues like keeping a copy of the markup text base-encoded in some html attribute which can be lead to unintentionally leaking full text while forwarding part of the message.
https://github.com/adam-p/markdown-here/issues/577