One of my favourite Mozilla Firefox extension is Live PageRank. This extension will automatically check and show the pagerank of the page you are browsing at the lower right conner on your Mozilla Firefox browser.
Previous version does not work in Firefox 3.5. However, the recent upgrade, version 0.9.7 is now supports Firefox 3.5 and Firefox 3.6. I am very happy with the upgrade as this extension is very useful for me to check my blogs pagerank.