The fun thing here is that as soon as I read Firefox, I knew it was going to be the split ligature issue. There are so few text rendering issues in Firefox, and that’s probably the only one most people will ever encounter.
My reaction to this was how would this ever be a common issue? When would you naturally have a line break in the middle of a ligature.
The screenshots show the answer: auto hyphenation. But at the point you're dropping a hyphen between the elements of a ligature, how do you not break up the ligature into two independent glyphs?
I assume the basic issue is that by the time they've decided how big the text is (and therefore if/where they need hyphenation), they've already decided to use the ligature.
Dynamically deciding whether or not to use the ligature when its at the linebreak seems expensive... refusing to do a ligature in the presence of a soft hyphen and otherwise treating the ligature as a single unit that cannot be broken seems like the more tractable way out of the problem... but I'm sure I'm avoiding or unaware of some additional piece of complexity.
