Lower-case f is an inconveniently-shaped letter. In order to stop the top-right-hand side of the letter bumping into a tall successor you need special ligatures in English at least for ff, fl, fi, ffl, ffi and those five are sufficiently venerable to appear in the Alphabetic Presentation Forms block of Unicode.
What about Irish and Gaelic, where f lenites in many sentences to fh? In my old Flamingo copy of The Best of Myles, there is no special ligature for it, so there’s a huge gap in the middle of the word seanfhocal “old word”. How does it look in your browser?
seanfhocal, fhàgail, fhèin
The two easy options in hot metal printing are to make your own fh or to choose a typeface where the widest part of the f is the crossbar.
Digital typography gives us another option, which is to simply print the h over the f’s ball terminal, which is a terminal in the shape of a ball, though I do keep misreading it in my head as being like “poet laureate”. How bad this looks depends mostly, I think, on whether you’ve had this drawn to your attention (sorry) and how new your spectacles are.
Scottish Gaelic provides another difficult letter combination:
fàs, fàinne, fàilte
This deserves a ligature of its own in fonts where the top of the f overshoots.
See also: Typography Deconstructed’s Type Glossary.