Here's from mozillazine: Some Firefox users report that they are unable to install or update themes and extensions because the download fails with the message, "Download Error -228". As a workaround, download the .xpi or .jar file to the desktop (or other location) by right-clicking the file link and selecting "Save Link As..." and then manually install it, either via "File -> Open File" or by dragging the file icon from the desktop onto an open Firefox window or to the Firefox Add-ons (Themes or Extensions) window