I was having an odd issue the other day, that I now solved by comparing the settings on a working Firefox profile with mine. For some reason, on one of my multiple Firefox profiles a list of web apps wasn't working. Among them were Discord, Miro Board, and Google Pay. I was suspecting that they all had the same issue, but didn't know what it was at that time. The security settings were the obvious suspect (Ad-blockers, DNS, HTTPS enforcement etc.) because clearly something was being blocked, however nothing helped. The solution was in the Network settings. (See screenshot) I was using Manual proxy settings from the time I messed with I2P networks (or something similar). I am not sure why having a proxy to localhost (127.0.0.1) on HTTP would somehow cause very specific major apps to stop working, while others keep working fine, but it happened to me anyway. For anyone in a similar situation, with those particular apps not working, check your proxy settings.