https://addons.mozilla.org/extensions/moreinfo.php?application=firefox&id=1471
RankQuest Toolbar for Firefox.
Just downloaded and has given me some great insight into my pages.
May be intersting to some here,Rankquest SEO(Search Engine Optimization) Toolbar provides you quick access to more than 25 intuitive SEO tools. Once you download and install the SEO Toolbar you are only one or two clicks away from carrying out most of your day to day SEO operations.
Dan