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An Alexa toolbar for Firefox? Yep, there's now an official one from Alexa called "Sparky". View traffic trends, reach and rank, while feeding traffic data back to Alexa.
There are also some unofficial Alexa toolbars for Firefox, as mentioned previously. The addition should quell concerns that sites popular with Firefox users are under-represented, although Alexa stats should always be taken with a pinch of salt.