Firebug is a great add-on to work with firefox.
Firebug a very very useful tool for web designers and developers to understand the coding behind the web page.
Basically by using this addon you can view the source code of a block, textbox or any element on a page.
And the BEST thing is that you can EDIT THE CODE directly there only.
You can download and install this add-on from the following url
https://addons.mozilla.org/en-US/firefox/collection/firebug_addons
After installing firebug, restart your browser.
Now right click on page, you will see a one more option “Inspect Element”
Now when you will click on Inspect Element then it will show you a screen, which will display the hierarchical structure of all the source code. You can see the code of any element.
Firebug Home page : https://addons.mozilla.org/en-US/firefox/collection/firebug_addons