I used to use the the Web Developer extension for Firefox when writing CSS. The only feature I used was the feature to display a red box around your different elements to see how changes affects them when there is something wrong. But I have always seen this Add-on a bit clumsy for this purpose which almost always leaves me with setting a red border for all the elements that I want to ”debug”, and that’s even more clumsy!
Hopefully these problems are at it’s end now! Yesterday Magnus showed me Aadvark. This Firefox Add-on lets you display different elements by hovering them with the mouse.
Try it out!
ps. If you are on Firefox 2, you might need to use the Nightly Tester Tools to be able to install Aadvark.