I looked at a previous thread on this - back in 2014, so I hope someone has an answer now. After downloading Firefox (for privacy reasons) all my existing .pdf files became Firefox .html files. Including .pdfs that I myself created by scanning documents. I have now been into the firefox preferences to ensure (I hope) that .pdfs are opened using Acrobat in future. But how do I get my existing files turned back? At the moment I feel very much like Joe (last thread in 2014) - Firefox should warn of this before downloading. One does not expect a Browser to change existing file types.