I recently installed Firebug to debug some CSS problems, but the only CSS information I got was the message “Unable to show styles”. The link in the message to the FAQ was not very helpful, either.
Fortunately, this post by Ingo was, as it solved this problem.
The problem is caused by a couple of ancient files under the Mozilla Firefox directory belonging to an ancient version of DOM Inspector, which seem to have remained there since the earliest version 1.0x of Firefox, and are easily identifiable by the file date (in my case, the year 2005).
To solve the problem, uninstall Firebug, remove the offending files, restart Firefox and reinstall Firebug.
The files are:
components.ini defaults.ini chrome\inspector.jar chrome\pipnss.jar components\inspector.dll components\inspector.xpt components\inspector-cmdline.js defaults\pref\inspector.js extensions\Extensions.rdf extensions\installed-extensions-processed.txt res\platform-forms.css res\builtin\platformHTMLBindings.xml res\inspector\search-registry.rdf res\inspector\viewer-registry.rdf
(list taken from above link, but as I said, you find the outdated files by their age)