It's "usq"; that's a medial S (ſ) rather than an f. It's short for "usque", meaning "up to" or "as far as".

A rough translation of the sentence would be "The experience teaches you as far as it can; the author can't say beyond what they've been given to speak."

“usque” (see <http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%...), because the second letter is a long s (“ſ”), not an “f”, and “q;” is the abbreviation for “que” as mentioned in the text:

That semicolonish mark at the end of the fourth line from the bottom isn’t a semicolon, it’s an abbreviation for que, Latin for “and.”

It's "usq", with a long S: https://en.wikipedia.org/wiki/Long_s - so it's an abbrevation for "usque".