Download the latest .xpi and install it in Thunderbird via Tools → Add-ons and Themes → gear menu → Install Add-on From File…

waiting

Target is Thunderbird 128+ (Manifest V3). Unsigned — Thunderbird installs it as-is (xpinstall.signatures.required defaults false).

Status: inline Source (HTML) mode works — reflow, 80-col wrap, formatting tools and Ctrl/Cmd-B/I/U/K, undo-safe. Markdown is scaffolded (not yet wired).