How to Debug Javascripts - Part2 (Firefox)
Firebug is not only a javascript debugger, it is a complete web debugging tool where you can explore the html DOM and much much more. And it is very easy to use. Once you install Firebug you will get a menu item in under the Tools menu. Using that you can enable and disable firebug for selected sites.
Once you open the Firebug you can inspect the javascripts under the Script tab and do the debugging. You can put breakpoints just by clicking the near the line number.
One special thing with Firebug is that you can put conditional break points. To add conditional break points just right click near the line number you want to put the break point and type your condition.
When there are Javascript errors in the page it is shown in the Firefox status bar. Once you click on that it will open all the errors in the console with possible reasons for that error. This will be very useful for you to find bugs in your application.
You can get more information on Firebug for Firebug site.