Have you ever clicked on a link in Firefox that's supposed to open an external application but it can't for some reason? I recently encountered this problem when clicking on a link for TVUPlayer for Mac. The error message reported:

Firefox doesn't know how to open this address, because the protocol (tvu) isn't associated with any program.

I tried the same thing on Firefox in Windows however and got a helpful "Choose Application" dialog that allowed me to select the correct application to open by browsing to it's location on my hard drive:

The problem is when Firefox is asked to open something outside of the browsing environment on a Mac, it often needs a protocol which instructs it how to. This should be something that's easy to address in Firefox Preferences but unfortunately it's not. You have to access Firefox's hidden settings to fix the problem.

To do so, type "about:config" in the URL bar:

You'll probably receive a warning but just click "I'll be careful, promise!". What you then see before you is Firefox's "hidden" settings. CMD-Click on any of the entries in the list and select "New" and "String":

In the pop-up box that appears, type "network.protocol-handler.app.myprotocol" replacing myprotocol with the name of the application you want to open which in my case is "tvu" i.e. "network.protocol-handler.app.tvu":

In the second dialog, enter the location on your hard drive where the application can be found. On Windows, this is usually something like "C:\Program Files\TVUPlayer". On Mac however, I've struggled to workout how to define the absolute path to my hard drive and after several failures, I'm stuck with "/Applications/TVUPlayer". There's obviously something that goes before that in the path but I can't quite work it out (sorry Mac users, maybe you can help here!).

You'll now see your spanking new String listed in the list:

That's it, you're done! When you next click on a link for the application in Firefox, it should open immediately or at least request if you'd like to open it.