I use Firebug and IE Developer Toolbar all the time to debug tricky CSS problems. But occasionally, a tricky bug comes up that only appears when you go to print the page.>
I use Firefox and the developer toolbar.
I use the real time css edit tool in the task bar, it's pretty usefull to modify your css on the fly on see the results in real time.
I also use the outline feature, which the div and stuff like that on mouse over, on your website. Really helpful to find div.
For the print problem, go to CSS -> Display CSS style by media -> print
There are a lot of other tool available in this one, I probably use about 10% of it.
Try maybe you find something usefull.