Q:

What's the meaning of a note before the time signature?

I'm working on a project using music sheets, but I don't understand music theory or notation.  
The sheet I'm studying is The Four Seasons, from what I gather the sheet was written with the musescore software (the accompanying MIDI has the name of the music and author at the meta event 09 and 10).  Before the time signature for the piece there's a eighth note with the fortissimo mark.
There is a meaning for the note?

A:

It's an engraving mistake.  The time signature should come straight after the key signature, before the first note.
Here's the opening of the Ricordi edition, from IMSLP.   The f indication is in brackets, this indicates that's it's an editorial addition, not in the original manuscript.
http://imslp.eu/Files/imglnks/euimg/f/f2/IMSLP386586-PMLP126432-Vivaldi__Antonio-Opere_Ricordi_F_I_No_22_scan.pdf

