Wasn't xulrunner based on XPCOM which Mozilla deprecated? I think this is one of the technical reasons for its decline, but I think the biggest reason was timing.
When xulrunner was being developed there wasn't the same interest in desktop software. It seems like Chromium reached a point where you could build desktop-class software, but the distribution model wasn't right. Someone then decided to adapt Chromium to solve the distribution problem and you have Electron.
If Firefox had evolved to allow the tipping point experience that Chrome did, then you might well have seen xulrunner be that thing. It was years ahead of itself, but perhaps you could question the focus of the folks on the Mozilla side who were spread across so many concepts/ areas compared to the folks at Google - but I think that's for a different thread.
When xulrunner was being developed there wasn't the same interest in desktop software. It seems like Chromium reached a point where you could build desktop-class software, but the distribution model wasn't right. Someone then decided to adapt Chromium to solve the distribution problem and you have Electron.
If Firefox had evolved to allow the tipping point experience that Chrome did, then you might well have seen xulrunner be that thing. It was years ahead of itself, but perhaps you could question the focus of the folks on the Mozilla side who were spread across so many concepts/ areas compared to the folks at Google - but I think that's for a different thread.