I have tried again and had no any problem downloading rmtp mp4 video from cta.ca with Replay Media Catcher.

Web Stream Dumper only works with HTTP streams. Please make sure "Capture RTMP streams" is checked.

It is also important to start recording with Replay Media Catcher and then and only then open your web-browser and navigate to a web-page with your a video stream.

p.s. And that's smart of you to use Firefox instead of Internet Explorer.