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.