I believe that you are mistaking ſ for F. ſ, also known as the long s was a form of the letter s used where s occurred at the beginning or in the middle of a word. Wikipedia has some more information on this.

Long story short: in older scripts and typefaces, there are two firms of s, long and short. The long s looks like an f without the cross, depending on the script/typeface. The long s was originally used in all cases except the ending of words, where the modern s was used. IIRC there was only ever one capital firm of s (S). This is all similar to the letter sigma in Greek, which had two forms used in the manner described above.

At some point, the long s fell out of use.

Source: I can dig one up if required, but I'll need to ask my paleographer wife.