Whether you are a professional web dev or just a beginner, if you are not using web-dev add on for FireFox you are wasting your precious time:

Here are just a fews of its features:

• Disable jave/javascript/meta redirect/referer/pop-up blocker/page colors...
• Disable cookie/delete cookie/add cookie/view cookie info/clear session
• CSS: disable/view/edit/etc....
• Form: display details/show passwords/.....
• And much more, I'm too lazy to type them out here

I have been using it for only 1 month now, and it's fantastic. If you haven't tried it yet, GET IT RIGHT NOW.
http://chrispederick.com/work/webdeveloper/

PS: some mod please make this post sticky, it will save people tons of time

Oh, and when you are at it, don't forget to get Firebug:

Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.

https://addons.mozilla.org/firefox/1843/

Actually I think FireBug is far more useful than the dev bar. In fact, with FireBug, the only thing I use the devbar for any more is to tell me at a glance whether a page is in quirks mode or not. But I think a lot of that has to do with which style of user interface you prefer - they're both great. FireBug is just more to my liking.