Using ProFont (http://www.tobias-jung.de/seekingprofont/) on OSX/Intel, typing "fi" results in the creation of a ligature that takes one character width. See attached screenshot.
I think that monospace fonts should not have ligatures...
In Latin script, I agree that monospace fonts should not have ligatures. In Indic and Arabic scripts, I'm not so sure. Maybe each cluster should occupy one character-cell, but maybe life isn't that simple.
The ligature must be provided by the font, yes? Unless we're generating the ligature somehow, this is a bug in the font, not in our font-rendering.
I'm of the opinion that this is a font issue too.
Possibly related (since I also use Profont for monospace) is that some character combinations, like lower-case-L, followed by forward slash: l/ simply disappear. These behaviours (the f-i ligature and the disappearing l-/) are not seen in Safari 3 beta using the same font + size, so if it is a font issue, Safari must be dealing with it differently than we are.
Safari doesn't support ligatures or kerning, so you wouldn't see it there.
How many of these broken fonts are there? We could work around this fairly easily, at least on Mac.
Given Pav's point in comment 7, this may really be a profont only problem. Part of the reason there are multiple people encountering it is that profont is a popular geek font for displaying monospace text legibly at small point sizes, as a way to fit more into a given space. Obviously we shouldn't work around a profont specific issue, but I don't know how widespread it is. I certainly haven't seen the behaviour using the default monospace fonts on either mac or windows.
http://www.google.com/search?q=profont+ligatures reveals that Firefox trunk isn't the application that has trouble due to ligatures in profont.
Yeah, ProFont. We can't really do anything about it -- the underlying text system is doing the ligaturization, not us. Use ProFontX.