Firefox Extensions will save your life someday
Well, they'll save you tons of time and a couple of headaches anyway. There are some really amazing extensions to Firefox that can make your development go so much smoother, especially in the QA phase or all the time for those of you that subscribe to Test Driven Development. When the Web Developer Extension by Chris Pederick came out in June of 2003, it was a God send. Never before had the web browser been so handy. Previously I had only cursed it, especially IE, which I still do actually - mostly IE6, damn you!
Now there are tons of extensions, most of which are useless, but there are a few good ones worth mentioning. So that you don't have to weed through the endless pages of add-ons to find the few good ones, here's the Short List:
Web Developer
Exposes everything about your HTML, JavaScript and CSS. The best feature is by far the ability to disable your cache.
FireBug
The QA person's best friend. Works a lot like the activity window in Safari, but on steroids. Great to see what your Flash app isn't loading correctly.
ShowIP
This one is really great when developing for a load balanced environment. Shows you the IP address of the current page in the status bar, nice!