If you are using NoScript you might encounter a certificate error when trying to download SeaMonkey from The SeaMonkey Project site. NoScript tries to force a https connection for a http only site. It is not a NoScript problem but how the Mozilla server, from which downloads are delivered, is configured.In this case please add download.cdn.mozilla.net to the NoScript “Never force secure (HTTPS) connections” options or turn off NoScript for this download only. The settings are located in the NoScript options under “Advanced → HTTPS”
Source: SeaMonkey News