I chose this One Rated 4 out of 5 stars

Gives all the info you need, just wish it would show it in a small, centered window instead of going to a website. Gives a button, but you have to place it on the toolbar yourself. A desktop version is available, I used that first and then installed the Firefox add-on actually. Unlike another reviewer I had no problem with it downloading something to my PC every time I used it.