I have been using the Web Developer toolbar for Firefox for some time now, but I recently learned of a new, powerful Extension, Firebug from Joe Hewitt. Firebug makes it very easy to inspect different portions of your code while on your web page. For example, click the Inspect tab and then you can see the related code as you hover over different elements on the page.
You can use Firebug as a part of your browser window (while you're interacting with it) or in its own window. the one drawback I see is that each time I change tabs, the new website is processed by Firebug; not that it's slow, but rather I'd like to see the window close or hide. One work-around is to go into the options and specify the websites that you want Firebug to interact with, and all others will therefore not inspected. You can also set what sites shouldn't be inspect. Perhaps a complex page, such as I use for posting to this blog, causes Firebug (and therefore Firefox) to crash. Just disallow that page and the problem goes away.