Firefox "Script error" in resource://gre/components/nsLoginManager.js

Question

Since yesterday I added a little function to one of my sites, to send me all the Javascript errors, that the clients get. It looks like this.

window.onerror = errorHandler;
function errorHandler(message, file, line) {
// send to server
}

For one of my users I am getting this:

message: Script Error.
file: resource://gre/components/nsLoginManager.js
line: 0

The user agent is Mozilla/5.0 (Windows NT 5.1; rv:23.0) Gecko/20100101 Firefox/23.0. The error happens on every page that is loaded, but only for one user, although many others should be using the current Firefox. I also tried it myself in Firefox 23 and did not get the error.

As far as I found out yet, the nsLoginManager is the firefox reponsible for the loading and storing of passwords in firefox. So two possibilities came to my mind:

1. The user has a thirdparty password manager installed
2. An exploit might have taken over the users password manager

Does anyone have other or more detailed ideas on this?