…Debian testing is continuously screwing the way Firefox (Iceweasel, whatever) is displaying fonts. The usual suspect is Pango, but I don't want to know about details.
Read it here
The simple fact that typographical ligatures were invented (nobody remembers why, possibly to spare some lead when this is how the printing was done) doesn't make them a useful invention! Human slavery was invented, and this didn't make it good. The Inquisition was invented, and this didn't make it good.