Earlier quoted context omitted.
Thunderbird didn't lose to an Electron-based app, it lost to Gmail.
Yes, but we're talking about XUL the platform. That platform was built with the same use-case in mind that Electron has: allowing people to build cross-platform desktop applications with web technologies (html, js etc). Unfortunately Mozilla failed to execute on that vision, leaving the field open for Electron and friends to emerge. Part of that failure was due to bad technological choices (RDF-XML was terrible ), pa…
Additionally, XUL uses web tech, but it is not a standard and was never intended to be a standard (not even an informal specification exists AFAIK), so it's a stretch to say that Mozilla failed to execute on pushing XUL. What would the web gain by pushing it?