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 (
f) connect to each other and letters like
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.