[This started as notes to myself to help clarify the problem and solution. It’s probably more suited to a Github issue than a blog post, and it may get copied into one.] # The story so far The Firefox and Thunderbird versions of Markdown Here both used nearly the same code – an old-style XUL extension. Tb is only capable of using a XUL extension, while Fx supports at least three extension types: XUL-based, Add-on SDK (aka Jetpack, aka jpm), and WebExtensions.