This question came up before. Pineman gave a useful link. I went there. I was told that I have the plug-in version 126.96.36.19901 installed. Isn't that the latest? If so, why doesn't Firefox know that?
You might want to re-open the old thread and ask your question there as the people who were participating in the discussion won't be able to see your question.
I believe this is the one: https://support.mozilla.org/en-US/questions/925671?