The "download" button stopped working in FF 18 Rated 2 out of 5 stars

I like this add-on and used it for years, but unfortunately with Firefox 18, it stopped working properly.

While the "open" button still works, the "download" button stopped working. All I get is the dialog to select the save folder, but then an error pops up saying the download failed. After that, another (Firefox) Download dialog (Run/Save choice dialog) opens, but even if I select to download the file in this dialog again, the file is not saved.

A fix would be greatly appreciated!