Firebug is quite possibly the ultimate web developers tool when it comes to "on the fly" inspection of code. I have been using this amazing Firefox addon for well over a year and during that time, it has not just saved me time, but has helped me become a much better developer.
Simply put, it's an on-the-fly code inspector for CSS, Javascript etc that allows you to view and edit code directly from your Firefox (or Chrome) web browser. See what your code changes do in real-time before going and actually touching the code.
See below for an example of the Firebug interface:

There are a few ways you can get Firebug. The first is direct from the Firebug Website, where you can select from a number of different versions. You can also get it direct from the Firefox or Google Chrome addon depositories. Simply search for Firebug and install as required.
Please leave feedback below if you have any questions about Firebug. I am always willing to answer any question you might have.
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