https://addons.mozilla.org/firefox/697/Displays formatted and color-coded source and optional CSS information for each element.
You can see exactly which CSS rules match for an element. The rules are displayed including file name and line number. The topmost element is that with the highest priority.
You can fold/unfold/hilite block elements (table, tr, td, div, span,...). Really cool feature: select a block level element of interest directly in the page and view it's source! To help you to quickly analyze the source code you can view images directly from the source and add comments to folded block elements. The code view is based on the rendered document, so you will also see dynamically (by Javascript) created and modified html elements! Works also with frames and selected text.