Note version 68 is an ESR release. The problem occurs in newer (71.0) 
releases as well.

I just tested against my demo tracker with ff 71.0.

On display inside a pre.msg, I have css set to:

  white-space: pre-wrap;
  word-wrap: break-word;

so even the one long line is displayed wrapped and I don't get
a horizontal scroll bar. The file db/file/msg/35/35529 does have
one long non newline terminated line.

This does bring up an issue where an additional class may need to be 
added to the msg.item.html template to allow the original formatting of 
the message without whitespace wrapping to be viewable.

Question, if a text email arrives with one long line, does it display
properly in the web interface? Is it stored as one long line?

If so then how does this bug manifest differently than the one long 
line in email?