How to hook the download dialog in a Firefox extension?

I'm writing a Firefox extension and I need to hook the download dialog that comes up when the user tries to download a file.

In other words, I need a script-defined function to be notified instead of the dialog appearing as normal.

Where do I begin? I tried reading thr开发者_JAVA技巧ough the XPCOM API Reference... but this is the closest thing I could find.

You need to override nsIDownloadManagerUI (by implementing it).