Unfortunately for us, Mozilla has already dropped support for non-WebExtension addons. Only WE addons can be uploaded to Mozilla Add-Ons, and WE support is only available since Firefox 55.
Even if we create a XUL addon - it won't be signed by Mozilla and so cannot be redistributed unless users use developer builds of outdated Firefox versions (52-ESR will be outdated in less than a year) or forks like Palemoon. For these (advanced) users other means to access blockchain-based domains are available (e.g. OpenNIC DNS).