Symptoms
The K2 Worklist is empty and does not prompt for login in Firefox.
Diagnoses
Firefox 40 introduced new security behavior where the HTTP authentication dialog box can no longer be triggered by cross-origin iframes and other resources (see https://www.fxsitecompat.com/en-US/docs/2015/http-auth-dialog-can-no-longer-be-triggered-by-cross-origin-resources/).
Resolution
Add your app domain to Firefox's trusted sites or configure Firefox's 'network.auth.allow-subresource-auth' setting.