Use FireBug! It is great! It gives you a little bit more power than traditional debugging. http://getfirebug.com/errors
Status bar error indicator
On the right side of the Firefox browser's status bar you will see a
little green icon. This is Firebug's way of telling you that
everything is A-Ok. When that icon turns into a red "x", things aren't
so peachy.
Click the "x" to open the Firebug error console which will show you
all of the errors that have occurred on the page.
No error soup
Most browsers report errors by dumping them all into one big window
that includes the problems with every web page you've ever visited.
Firebug is kinder than that; it shows you only the errors for the page
that you're looking at.
Jump to the debugger
Every error report has a link on its right side that points to the
file and line number where the error occurred. Clicking this link will
take you right to the Firebug JavaScript debugger or CSS inspector so
that you can get started on solving the problem right away.
Some errors also include the actual snippet of source that contains
the error, which is also a link to the original file.