I have FireBug enabled if that makes a difference. (Nothing comes up in Firebug either.)
Although you have Firebug, you probably don't have it correctly enabled. Right click the bug icon and select Enable all panels and On for all websites.
You may also try the Error console which lies under the Tools menu.
Under the "Tools" Menu and select "Error Console" - hope that helps
perhaps your code does something that doesn't throw an error in Firefox (like string[indexing])
FireBug won't pop up when there's an error, either. Instead, it'll show up in the Console tab assuming it is enabled in the way described by Miguel earlier.