…only does it on a mac.
That is interesting. I do not use a mac, so if a mac does that automatically it means that a font needs to have fi and fl ligature glyphs mapped for the mac platform, even if the font does not have an overhanging f and there is no need for fi and fl ligatures for the font on typographic grounds!
An interesting aspect of the fi and fl glyph positions in the new font template of Font Creator 5.0 is that they are within the first 256 glyph positions. I think that this is important. I like to produce all seven of the U+FB00 to U+FB06 ligatures in my fonts, yet resisted moving the fi and fl to the end with the others! One font mentioned below (Quest text) has the glyph twice so that the glyphs are all together near the end as well as the mac versions not being moved, but that is due to the history of the Quest text font, which was originally produced using the Softy program with which I started making fonts.
If you would like to try an experiment, my Quest text font has a noticeably different glyph for the fi and fl ligatures as compared with fi and fl from ordinary letters: this was so that transcribing 18th Century books would clearly show the presence of a ligature, so if you were to try Quest text on a mac and a PC, it may be that the mac would use the ligature automatically whereas the PC would not.
My 10000 font has ligatures for fi and fl even though there is no display difference as compared with fi and fl from ordinary letters. I put them in as a sort of aesthetic and artistic whim as if from the days of metal type, now reading your post it seems that at least for fi and fl that that may be a practical necessity if anyone is to use my fonts on a mac and needs to write about fields flooding and kingfishers flying!
It looks like I need to go though my fonts and make sure that there are fi and fl ligatures mapped for the mac for all of them.
Does the mac try to use fi and fl ligatures by default on all macs? Is it a feature that can be turned off?
William Overington