You can add firefox browsers to the list of browers it acts strange with. Got the error:
An error occurred running the Unity content on this page. See your browser JavaScript console for more info. The error was:
TypeError: rules[r].selectorText is undefined
_newgroundsioAddPluginScripts@https://uploads.ungrounded.net/alternate/1538000/1538279_alternate_113862_r22.zip/Build/Web.framework.js:2:303468
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unityFramework/Module.dynCall_iiiii@https://uploads.ungrounded.net/alternate/1538000/1538279_alternate_113862_r22.zip/Build/Web.framework.js:2:492459
invoke_iiiii@https://uploads.ungrounded.net/alternate/1538000/1538279_alternate_113862_r22.zip/Build/Web.framework.js:2:356101
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unityFramework/Module.dynCall_v@https://uploads.ungrounded.net/alternate/1538000/1538279_alternate_113862_r22.zip/Build/Web.framework.js:2:501798
browserIterationFunc@https://uploads.ungrounded.net/alternate/1538000/1538279_alternate_113862_r22.zip/Build/Web.framework.js:2:170874
runIter@https://uploads.ungrounded.net/alternate/1538000/1538279_alternate_113862_r22.zip/Build/Web.framework.js:2:173947
Browser_mainLoop_runner@https://uploads.ungrounded.net/alternate/1538000/1538279_alternate_113862_r22.zip/Build/Web.framework.js:2:172409
On the plus side, I played it in Chrome and it's pretty good!