If you are getting the following JavaScript error in Firefox 3, here is a quick workaround:
Error: invalid 'in' operand this._sandBox
Source File: file:///C:/Program%20Files/Mozilla%20Firefox/components/nsProxyAutoConfig.js
Line: 95
Open the file mentioned in your favourite text editor (one that can handle unix line endings; if you have nothing else, use WordPad), and seach for the this line:
if (!("FindProxyForURL" in this._sandBox))
return null;
and wrap it in a try/catch block like this:
try {
if (!("FindProxyForURL" in this._sandBox))
return null;
} catch (e) { return null; }
Save your changes, and restart Firefox.
Although this is probably not the correct way to solve this problem, at least it helps to keep the 'Error Console' garbage free.