Rated 5 out of 5 stars

This can be fixed for later versions of Firefox by changing (this will probably lose formatting):
if (browserHistory.isVisited(download.source))

to:
Cc["@mozilla.org/browser/history;1"].getService(Ci.mozIAsyncHistory).isURIVisited(download.source, function(uri, visited) {
if (visited)

and after the "else" statement in that block of code, add:
});

It shows up two times.

This review is for a previous version of the add-on (1.2.1).  This user has a previous review of this add-on.