I've given up on using the italic fountain pen as a highlighter for a couple reasons. The first is that the nib on the pen is too narrow and its squared-off shape makes it too hard to lay down a stroke of highlighting ink. As I pull it across the text, it highlights only about half the line, and if I'm not very careful I get skips in the line. The second is that the Firefly ink, while initially fluorescent, seems to fade to ordinary yellow after a few days. Coupled with the narrow lines, it makes the highlighting difficult to see.
I haven't entirely given up on the reuseable highlighter idea, though. Bic makes a line of liquid-filled highlighters called the "Brite Liner" series which I quite like because I can see when one's about to run out. The next time I empty one, I'll probably experiment to see if there's a way to refill it with the Firefly ink.
I wonder if someone makes felt nibs for fountain pens.
Update: follow-up here.