There is a bug is in viewing css style sheets with the Firebug plugin when you have a installed version of Firefox that was updated from v1.5x to version 2.x.
To fix this simply delete the following files from the “components” folder in the main Mozilla Firefox folder:
inspector.dll
inspector.xpt
inspector-cmdline.js
Then reinstall Firefox 2.x OVER the previous install – you do not have to uninstall Firefox.