I had a couple of .mht files which opened in IE, a browser I never use. I renamed their extension to .HTML to run it in Firefox. When I opened the file in Firefox, Firefox created new tabs endlessly. I had to close it to stop it.

Is this a security flaw in Firefox?

  • 5
    Even if not a security flaw, it's a bug and submit it to FF's bugzilla Commented Jan 14, 2013 at 7:39
  • It sounds like more of an annoyance rather than a security flaw.
    – Hammo
    Commented Jan 14, 2013 at 8:48

1 Answer 1


Firefox does not support MHT files without addons.

It is possible that you have tricked Firefox into opening each MIME object from the MHT file into a new tab. It is doubtful that this could turn into a security flaw.


I confirm that behavior with a valid MHT file. It is a Firefox configuration issue. By default, Firefox will not recognize the MHT and will ask you for a "Open with" program. If you select to open with Firefox and check "Do this automatically for files like this from now on", then it goes into a loop and will open many new tabs. It will stop if you press the Escape key.

This Mozilla support article is about the same symptom and contains a configuration fix.

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