Web Developer — adds a robust set of tools to peek under the hood. Adds a toolbar to your windows to let you do things like disable Javascript, cookies, css, see field names for a form on the page etc.
Firebug – the king of web developer firefox plug-ins.
View Source Chart
Ever find the “View Source” that Firefox provides a little hard to read? The windows opens with a raw display of the HTML as rendered by the page, but looking at raw HTML can be straining to the eyes if not downright dizzying. When working in code editors, the editor typically does syntax coloring. Here’s a great Firefox add-on which adds a “View Source Chart” to every page so that your view source comes out nicely formatted and colored:
