I don't know what would break tab pinning, but it is working for me. It may be specific to Windows or Win 8. Could be a bug. The closest "bug report" i find is someone claiming that... pinned tabs should always change content when a URL is opened while that tab is displayed! https://bugzilla.mozilla.org/show_bug.cgi?id=908968 Although in some cases, it seems that this behavior can occur and is not yet entirely fixed: https://bugzilla.mozilla.org/show_bug.cgi?id=579874 Can you try creating a new profile, and see if the behavior persists? Type '''about:profiles''' in the address bar, and Enter. Click ''Create a New Profile'' Give it a name. Do not "choose folder" Click Finish Scroll to the new profile Click ''Launch Profile in new Browser'' Close the original browser window Test the pinned tab behavior. 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./

Ha! Good timing there. A moderator might delete your post. Or you can restore your original question and answer it yourself, marking your answer as the solution. Other people can find that helpful!