Consider migrating it to use WebExtensions.
Consider migrating Thunderbird extension from legacy[2] to webextension[1]. From release of Thunderbird (i.e. 63) have support for webextension[3] while maintaining legacy and hybrid addon support. But it would be nicer to support for webextension because sooner or later they will deprecate it.
[1] : https://wiki.mozilla.org/Thunderbird/Add-ons_Guide_63
[2] : https://wiki.mozilla.org/Thunderbird/Add-ons_Guide_57
[3] : https://thunderbird-webextensions.readthedocs.io/en/latest/