I Use Web Developer and Firefox Developer Tools
Tried Firebug. Prefer the Web Developer plugin. Firebug didn't suit my preference for editing CSS and HTML freestyle as though using a text editor. Maybe I just couldn't be bothered to learn how to do that with Firebug; is it possible to do with Firebug? It can be done with Web Developer.
Love the Firefox inbuilt developer tools, especially the 3D cube and the CSS viewer. I would drop Web Developer altogether if I could use the inbuilt FF developer tools to edit HTML without needing to double-click and right-click the editor in order to make edits.
Edit:
Just noticed I can 'Edit as HTML' with FF Developer Tools but this is still not as free as scrolling through the HTML and editing the HTML as though using a text editor.