Firefox updated itself to version 66 and then removed many add-on including Key.
Are you updating Key to support new Firefox add-on policy?
I am only an F-Secure user (their home solutions).
There was a discussion about subject (probably):
Thus, I think that it is trouble on Firefox side. And, perhaps, all is OK with F-Secure addons (but my opinion is that it is good to doublecheck it, anyway).
And - based on web - Firefox already pushed some updates and quick fix.
Another URLs and more information:
Does your experience is about this situation? OR your concern is about another situation?
You are right, the problem was in Firefox. New update to Firefox game today and now the add-on is working again.