It seems that MuseScore ignores OpenType substitutions. Free Serif, which is a fully-loaded font (more so than most) has a complete set of f ligatures, but they don't show up in MuseScore. Other than fi and fl, there are no keystrokes that will allow a user to access the ligatures ff, ffi, and ffl. See attached screen shots, one showing the word Affettuoso as it appears in MuseScore, and the same word (plus a few others) in Adobe Illustrator. Both examples use Free Serif. Is there a setting that needs to be turned on? As you can see, Affettuoso does not look good without the ff ligature.