By the way it is not the letter F, its called a long S, it is not only from old french writtings but also english, Italian, spanish and more.
If you look it up every language has a set of rules about the uses of the long S. For example im posting 2 images of the rules for the Long S in french and in English.
Attachment 179095Attachment 179096
I don't know however if there were a difference of pronunciation, still it is an interesting clue to help with the datation of vintage french documents.[/QUOTE]