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.

https://github.com/adam-p/markdown-here/issues/577

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.