Firebug is a fantastic Firefox Add-On which allows realtime viewing and editing of a page’s DOM, CSS, etc. as well as net usage reports, and even JS timing and profiling.
If you work on the web, it’s well worth your time to familiarize yourself with this tool.
Here’s a video presentation: YUI Theater: Joe Hewitt, Welcome to Firebug 1.0 ยป Yahoo! User Interface Blog – summary
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