I just removed Amazon Assistant from a friend's computer. It pops up on various sties, e.g. eBay and covers part of the page. Click the X in the upper right corner and nothing appends. It follows her around and tries to redirect her to Amazon.

SPAM! it pops up on any and all sights.

To get rid of it on Firefox go to Tools> Add-Ons> click the three dots menu to the right of Amazon Assistant and remove it.