Are you saying iTunes won't open when you try a link to the app store (etc.) on a web page in Firefox? Does it matter whether iTunes is already running when you click the link?

I'm having this issue as well. Anytime I click on items that would normally open in Itunes, it tells me I need to install Itunes. I've uninstalled/reinstalled Itunes to no avail. In looking in Firefox's preferences, there's no reference to Itunes in the Applications tab.