You need a Developer Toolbar (or similar) with Script console to study these errors. There are some pretty extensive (complicated) developer toolbar plugins for FireFox but I prefer the one that is already integrated in the Chrome or (even better) in the Opera browser.
Then right-click anywhere on the page and select Inspect Element. This opens up the developer tools from which you can switch to a Console tab or (in Opera Dragonfly) to the Errors tab. Then reload the page and watch the errors streaming by. Sometimes this immediately points you in the right direction, other times they are too vague but then Googling might help. :)
Another fantastic feature of the Opera browser is that if you right-click on a web page and choose "View page source", you can then edit that source and apply the changes to see the result immediately. Great for testing any theories by simply removing the suspected offender -- in your case the line that included the Event Manager script...
So, short answer: get the Opera browser. A fantastic browser in itself (fast, modern, with e-mail suite) with amazing developer tools.