Edit your document on-the-fly through FireBug

Firebug
Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page
Firebug Window Overview:
* Console Tab: Contains command line javascript, shows javascript error message log, can enter JavaSript commands after the >>> at the bottom
* HTML Tab: Shows HTML as an indented hierachy of DOM nodes, which you can open and close to see or hide child nodes.
* CSS Tab: CSS inspector, view all loaded style sheets, modify styles on the fly. See list of styles sheets and select one to view from this pane by clicking on the drop down list on the top of Firebug window, to the right of “Edit”.
* Script Tab: Shows the javascript files and the calling document. See list of included JavaScript files and select one to view from this pane by clicking on the drop down list on the top of Firebug window, to the right of “Inspect”. Set breakpoints and conditions underwhich break points appear.
* DOM Tab: Shows all the page objects and properties of the window. As variables are properties of the window object, Firebug displays all JavaScript variables and their values.
* Net Tab: Shows all the downloads, how long each resource took to download, the HTTP request headers and server response sent for each resource. The XHR tab is useful for AJAX debugging.
Edit your document on-the-fly.
You can change text nodes by selecting the text node with the “inspect” function and then clicking on the text node in the HTML panel.
Firebug is both an inspector and an editor. All objects in the HTML, CSS and JavaScript files can be edited with a single or double click. As you type, the changes are immediately applied in the browser window providing instant feedback. The DOM inspector allows for full in-place editing of your document structure, not just text nodes. Simply click on the “edit” tab next to the “inspect tab”, and the left panel becomes a black and white text editor. In the CSS panel, Firebug autocompletes as you type. In the DOM inspector, Firebug autocompletes property names when you hit the Tab key.