Firebug is very useful, and a real time saver, once you get the hang of it. It's definitely perfect for testing out different effects and other things before applying it permanently to your design.
By the way: while I think that Firefox 3 is very annoying (very buggy in my case), the Mozilla Firefox browser in general is the best choice for any web developer/designer. I suggest that you get used to making your designs look especially right in that browser (not saying that you should ignore the other browsers, though), since it's the most faithful to the web standards, judging for what I've seen and read. Internet Explorer is every web designer's worst nightmare (thus regular users suffer as well), so every time you can, try inviting others to use Mozilla Firefox.