I don’t know what the name of the file is, but I’m guessing it’s the path to your email client of choice. What’s probably happening is that the protocol handler for ‘mailto’ links is not properly configured in Firefox.
Does your error message look like the “Launch Application” dialog shown in this link?
https://developer.mozilla.org/en-US/docs/Web-based_protocol_handlers
If so, then I think maybe you clicked on the app you wanted (say, gmail) and then clicked “choose” instead of “OK” (maybe OK was still grayed out, which would mean it’s not set up properly, which would be wierd.)
If that’s not the case then I think you will need to mess about a bit with firefox config in order to get it to work. In Firefox, open the menu, choose Preferences, and go to the Applications tab. Type “mailto” in the search bar. Under Action there’s a drop-down menu, pick “Use Gmail” or whatever one is in there that you want.