I am using the legacy skritter site. Today the above message appeared. I have downloaded the latest version of Adobe Flash, I have enabled it in add-on manager in Firefox, have also selected ‘always activate’. But I still get the above message.
Does the message show in the canvas area (replacing where the Skritter canvas would be)? If so, I think you can click that area, which should have a pop up asking to allow Flash to be used.