Right click on the "details" at the top of the history items displayed, and activate the date/time detail by clicking on it. if it's already checked then you may just need to scroll left or right, or you can re-size the other details (by grabbing the separators and left clicking to slide them) so it makes room for date/time to be viewed without having to scroll. I've marked where to right click in the image attached. (Your version of Firefox will probably look a little different, but the options still live in the same place)

You can also do that via the Views button (second one) in the Library and set via "Show Columns" which columns that you want to see.