The following pyside snippet will NOT show the Fira Code OTF font when run on linux, but /will/ show it on MacOS (I haven't checked Windows yet).

from PySide2.QtWidgets import QApplication
from PySide2.QtWidgets import QFontDialog
app = QApplication()
QFontDialog.getFont(None, None, options=QFontDialog.DontUseNativeDialog | QFontDialog.MonospacedFonts)
sys.exit(app.exec_())

I have confirmed that Fira Code is a monospace font and is installed on linux via the following (also, it's usable in other apps that filter for monospace like the terminal):

$ fc-match -a monospace|grep -i fira
FiraCode-Regular.otf: "Fira Code" "Regular"
FiraCode-Retina.otf: "Fira Code" "Retina"
FiraCode-Medium.otf: "Fira Code" "Medium"
FiraCode-Light.otf: "Fira Code" "Light"
FiraCode-SemiBold.otf: "Fira Code" "SemiBold"
FiraCode-Bold.otf: "Fira Code" "Bold"

I've included the PySide2 for reproducibility but the same bug exists in the native APIs as well.  The bug is also there whether or not the QFontDialog.DontUseNativeDialog option is used.