The inability to use the instant pop up window on selected text (that disappears when deselecting text) rather than clicking an icon and getting a popup window is the main reason I don't use Firefox for the majority of my internet use. I want to be able to click and go.

Problem with scrolling behaviour in the popup. Also it doesn't remember the target language.

The UI works really bad (the window often opens outside of the visible area and other stressful little things), and it randomly ceases to translate at all, so I suspect that the developer abandoned this project. It's a pity, because with a bit of work, it could be a very good and useful add-on.

Great limitation: only translate TO english. In preferences there is no a way to set a different target language.