SEOpen has released the latest update that is now compatible with Firefox 3.5.3. Its my favourite SEO tool and I've been using it since 2005. It provides quick, relevant SEO information such as Google cache dates, backlink data and access to Whois. The MSN part of the tool has been updated to take account of the Bing rebranding.
To many SEO tools take time to load and require several key strokes. I find SEOpen the easiest SEO tool to use.