STR: 1) go to https://builder.addons.mozilla.org/package/183107/revision/2/ 2) click Test 3 times (this loads, unloads, and finally loads the add-on again) 3) dispatch the custom event on a window. Actual results: Error: TypeError: can't access dead object. Expected results: I'm not sure what actually throws that error, but the file and line number should be listed in the error message.

What actually throws the error is C++ code, where there's no file/line number available. You're trying to access an object from a window after that window's closed, which is generally a good way to "leak" lots of memory -- except that we interpose a wrapper between your code and the actual object from the other window, and when that window closes, it severs the link and makes any use of the wrapper throw the dead-object error. Probably some listener's not getting properly removed from the window when it's closed...or something. Note that the exception probably has a "stack" property on it, that includes call stack information along with file/line numbers. The console could easily display this property if it wanted. (If the stack's empty, the dead object's probably being used as a listener of some sort, and the attempt to call it is what's bailing.)

Is there really no way for the C++ code to report a useful error (source/line)? When one of these shows up in someone's console, I'd hope there was a better way to find what was triggering it than to hook up a debugger...

There's that stack property, right?

I didn't check, but even if it's there, getting it to the console through the nsIConsoleService and friends is far from trivial (bug 814497).