Wouldn't recommend this one. You either need to have it on all the time, which pushes the page down (not good for developers), or you need to have it set 'by request", with no way to then turn it on for each page (FF 3.6.13 - the single click on the Seoquake icon does nothing - no seo bar).
It's one of those extensions that may have potential, but the unintuitive way it works makes it more trouble than it is worth. I can't rate its pure SEO effectiveness as I don't trust it enough to keep it on all the time. The 2 stars purely because I want to save other peoples time, but have not tested it enough to warrant giving 1 star.
EDIT: I see that the turn on / off click icon issue is with the seobar function. with seotoolbar it works. My recommendation would be to include an option to turn on (when by request is selected) seobar or toolbar through seoquake option when right clicking. This is Windows standard.
At least I can use it now, and will revise this if my opinion of it changes.