KGen
- Display and analyse document keywords from sidebar.
- https://addons.mozilla.org/en-US/firefox/addon/4788

SearchStatus
- A tool for the specialized needs of search engine marketers. Shows ranks and information on sites.
- http://www.quirk.biz/searchstatus/

SEO Links Extension
- Tooltip enhancement that shows search engine link-popularity and ranking data of any link.
- http://www.webmasterbrain.com/seo-tools/firefox-extensions/seo-links/

SEOpen
- PageRank, keyword density, HTML validator, and other search engine optimization-related features made available via right-click.
- http://seopen.com/firefox-extension/