Rated 5 out of 5 stars

After AK49BWL's enlightening steps, it works on FF5 as a charm. To avoid cacheviewer opening in a new window go to about:config and change the extensions.cacheviewer.open_in_separate_window value from True to False. After that it will open in a new tab.

This review is for a previous version of the add-on (0.6.3).