With certain Windows fonts there is a display issue where ligature-kerning seems to flip out and make the two letters bold. Here's a screenshot of an email using Helvetica in Firefox:

As you can see, it seems to be something about the way crossed characters (t and f) connect to each other and letters like i.

I've had this problem for a very long time (over multiple systems) and wondered if anybody has ever seen similar issues popping up and found a way to fix them.

Note: I think this does just affect Firefox. Chrome and LibreOffice are both fine.