I've recently had to reinstall Firefox and I've noticed that iTunes wasn't
in the list of programs to subscribe to a feed with when viewing a feed. I
seem to remember that being there by default last time.
So I clicked on "Choose application" and added iTunes. The problem is that
this isn't working. iTunes is in the list of programs that I can use to
subscribe to feeds with, but when I click "subscribe now" all that happens
is that iTunes comes into the foreground.

I've discovered that to subscribe to a feed in iTunes, you need to open it
like this:
itunes.exe /url "%1"
(where %1 is the feed url)
but I haven't found a way of editing the setting in about:config to this
without itunes being removed from the list of available programs.

My iTunes is version
My Firefox is version (and the problem persists in a blank profile)

Can someone come up with a solution to my problem?

I hope this was easy enough to understand.