Just copypasta from my answer to your previous question: As for mht files and other useful things which have been rendered inoperative like UnMHT or Mozilla Archive Format (or Zombiekeys, FTP, Gopher, etc., etc.), these are broken by the browser changing to more secure Chromium-type extensions which cannot interact with the browser itself. Or simply because they dropped support entirely, but extensions can no longer make up for that. (And because they still have a wildly incomplete extension API.) I keep old copies of applications for these reasons. You can even get a portable Firefox or whatever (e.g., 52 or so, or the most recent that fulfills your needs). https://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20Portable%20Ed./