It appears that Pixel streaming does not currently work with the Mozilla Firefox web browser
Docs link: https://docs.unrealengine.com/en-US/Platforms/PixelStreaming/PixelStreamingIntro/index.html
Result: The Firefox browser never connects to the pixel streaming project
Expected: The Firefox browser would connect to the app that is using pixel streaming
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-80170 in the post.