Comment 5 for bug 605513

Oh, actually, we probably started throwing when the patch for bug 407359 landed. That patch assumes that mInput is a autocomplete.xml-implemented nsIAutocompleteInput, which isn't always the case (e.g. for content autocomplete, where we get the impl from nsFormFillController instead). The patch for bug 419725 just changed the error.

I suppose there a couple of ways to fix this - we could detect the underlying nsIAutocompleteInput implementation in that popuphiding handler somehow and behave differently based on that, or have content and chrome autocomplete use different popup implementations.