How to debug "uncaught exception: undefined (unknown)" in Firefox

Question

I have this line come up in the console, only in Firefox, from my JavaScript application I'm developing:

It seems relatively harmless, but I'm curious if there's any way to deduce its origin, because it must come from somewhere, even if it claims 'unknown'. Wrapping the entire script in a try/catch block and toggling Firefox's "Pause on Exception" setting doesn't do anything, which seems to imply it's a special exception? I have some ideas what parts of my code might be causing it that are using Working Draft APIs, but I'm more interested in why it reports this way and what I can do about it. Does Firefox not provide any more detail?