I would like to apologize with the author for having left a previous very rude and negative review, which I promptly deleted. Evidently the extension was not compatible with my Firefox version (some months old). I tried to update Firefox and now it works perfectly !!! A button appears on the upper left, it is enough to click, wait some seconds, then a print dialog opens. You have just to select "print to pdf" as printer device, and your calameo document is saved ! Great job ! If you are experiencing troubles, try updating Firefox first.