To login to the webplayer on Firefox I had to reinstall a language pack.
I had two different Windows computers with similar plugin profiles on Firefox. Webplayer worked on one but not the other. The only difference is one has non-English language packs installed. I removed the language packs and was finally able to login to the webplayer using Firefox. I was then able to reinstall the language pack and subsequently login again. Using version 48.0 of Firefox.