That message means the extension uses XUL's API or XPCOM's API, whereas
Mozilla now pushes WebExtensions, a technology it shares with other Web
browsers, what eases the work of extension developers. More importantly,
WebExtensions' API does not expose Firefox's internals as much as XUL/XPCOM,
what allows ground-breaking changes of the core (with XUL/XPCOM, extensions
can break because they are tightly coupled with this core). When the
extension is proprietary, WebExtensions somehow is better for the user's
privacy (the user get to accept/deny the access the extension wants to some
information).
See
https://blog.mozilla.org/addons/2015/08/21/the-future-of-developing-firefox-add-ons/
It may be that defining keyboard shortcuts actually requires accesses to
Firefox internals that WebExtensions does not provide anymore. If so, the
extension will stop working when the transition to WebExtensions will be
over...
