User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:89.0) Gecko/20100101 Firefox/89.0

Steps to reproduce:

Any open of .MHT file starts creation of infinite number of new tabs. Only way to stop is to delete Firefox in task manager. Had to use WORD to open .MHT files. I'm using Windows 10 on my laptop. Latest Firefox.

Actual results:

System locked with Firefox continuously opening tabs. Must shrink Firefox window and use task manager to close Firefox. Sometimes need several try to close.

Expected results:

File should open and display contents. Tried several .MHT files with same resulting bug.

The Bugbug bot thinks this bug should belong to the 'Toolkit::Startup and Profile System' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

You've probably set Firefox as the application to open .mht files in about:preferences. Change that entry to something other than Firefox to fix this

Yes, this happens when Firefox is set as application to open .mht files. Why is this a problem?
I'll keep using another app to open these files.

Firefox doesn't understand mht files so it hands them off to an external program, you've chosen Firefox as that external program but it doesn't understand mht files so it... etc