I can reproduce the error on your demo page. I think that is my firefox issue as I tried it on another computer and it's fine.

Two of the computers encouter the problem but another one is fine. They are all firefox.

 

following is the error I got from firefox:

 

Could not read chrome manifest file 'C:\Program Files\Mozilla Firefox\extensions\{972ce4c6-7e08-4474-a285-3208198ce6fd}\chrome.manifest'.