Since v1.5 it's IMHO best currency converter Rated 5 out of 5 stars

I was waiting for the several features to add, and today in the v1.5 there are enable/disabled on startup and in anytime without the need of reloading the webpage. And there is even an option to use statusbar icon!! IMHO it's most convenient currency converter for Firefox.

This review is for a previous version of the add-on (1.5).