F only makes one sound regardless of whether it is broad or slender and that is the F sound we are used to in English.
When it is lenited is looses all its sound, it just gives up and makes no sound at all.
And, it will lenite at the drop of a had. Really f will lenite when no other letter will.
Any word that has a lenited F in the front of it should be treated as if it begins with the letter that follows the lenited F.
For feminine nouns that begin with F the definite article would like to be a' +lenition but when F lenites and the sound goes away that doesn't work anymore because you end up with two vowel sounds together, so the article changes to an + lenition.