FireBug is an awesome Firefox extension for web developers.
The FireBug developers have just put out a 1.0 release and it includes loads of cool features. There’s one that I just have to share, though, and it is awesome.
If you use inspect to select an element on the page, you can then double-click on bits of the CSS in the right hand pane, and get this, edit them in real time! That means you can literally play around with how a website looks in real time and it’s really useful when you want to make a CSS tweak to something. You just use FireBug to preview it in real time and adjust your edits to make it just right, then you can tweak the actual file. No page reloading needed (or worse Ctrl-F5 to clear the cache).