You don't necessarily need Firebug to accomplish this. The built in tools with Firefox are really good now and come with a javascript debugger.
After you page loads, right click anywhere and select Inspect Element
. This will bring up the Firefox Inspector Tools.
For javascript click on to the Console
tab and then reload your page and it should show any errors that come from javascript. You cah also type into this console to test javascript on your page.
On the Debugger
tab it will show all the javascript that has loaded for your page and you can add breakpoints in here and then reload your page to test them.