Using Firefox is becoming more the norm with many internet users.One of the best things that you can do with the browser is add plugins and tools to make life easier. I have listed below several great Firefox plugins for SEO:

SEOpen - 23 different metrics from back links on Yahoo and Google to Archived pages, whois and checking robots.txt.
SEO For Firefox -Aaron Wall’s clever and configurable tool with 13 different signals ranging from domain age and Page Rank to del.icio.us bookmark and .edu inbound links.
yExplore - Thomas’ handy tool offering easy right click access to Yahoo Site Explorer info.