The folllowing is a list of useful add-ons that can be installed on Firefox:
SEO Rank Checker: – Easily and automatically track your rankings (and rank changes) on Google.com, international versions of Google, Yahoo! Search, and the Microsoft Windows Live search engine.
Greasemonkey: – Allows you to check if a link has a nofollow tag.
SEO Playground Tool: – This tool gives you lots of useful SEO related information.