If you have anything to do with CSS/XHTML and web design, Chris Pederick’s Web Developer extension for FireFox is a must have! It lets you do a whole bunch of things directly within FireFox, including:
– Display CSS stylesheets used by a web page
– Edit the CSS sytlesheets directly and see the web page change as each change is made
– Edit HTML
– Disable CSS stylesheets
– Display image file sizes, paths, dimensions & alt attributes
– View HTTP response headers
– Validate CSS, XHTML, RSS, links, etc.
– Show rulers, line guides
Not an extensive list of all the stuff that this great plug-in does. Download it for free from here and make FireFox an infinitely more useful tool.