1. Visually modifying page layout in Firebug
Firebug's Layout panel let's you to examine the selected element's height, width and other position related attributes in a visual manner. Apart from showing the positional attribute values, the Layout panel allows you to edit the values directly from the panel. To edit a value, just click on it and the modified value will be reflected immediately on the page.
2. See the computed styles for an element
As you might have known, CSS apply styles in a cascading manner (after all, it is cascading style sheet, isn't it?). I.e. The styles that are defined for the parent element will be applied to all of it's child elements too. But, the child element is allowed to override any of the parent styles and define its own styles. Thus, the final set of style attributes which are applied to an element is always changed at the run time. To view the final style attributes, which are calculated after applying all the inherited style definitions, are displayed in the Computed panel. Please note that it is not possible to edit any attribute from this panel.
3. Find out which CSS file is the black sheep
Some times it becomes very difficult to figure out which CSS definition is affecting a element style. When more than one CSS files are being used in a single page, then it becomes even more difficult. In such scenarios, Firebug could be used to identify the CSS file name from which an CSS style definition is being taken. For example, in the below screenshot you can see that the particular page uses two CSS files (screen.css and reset.css) and the Style panel clearly displays which style is taken from which CSS file. The overridden styles are striked out. The Style panel also links to the URL of the style file being used, so it is easy for us to get the entire CSS file if needed.
4. Not just edit CSS, edit HTML too
Firebug Style panel lets you to edit any CSS attribute and view the results immediately. Firebug can be used to edit the HTML content also. To do it, right click on the tag which you want to edit in the HTML panel and then select
Edit HTML (or, just double click on the element). Firebug opens the inner HTML of the element you selected in edit mode. The changes you make will be visible on the page immediately.
5. Jump to the element you are editing
This is a nifty option for quickly navigating to the element that you are currently working. While editing a HTML element, right from the HTML panel, you can easily bring that element into view by right clicking and selecting Scroll into view.