So I have my own Discourse based Forums, and I got a weird Issue that I was conveniently able to reproduce while making an Account over here too. To me personally it happened on Kubuntu LTS 18.04 with the latest Release of Firefox being installed (probably happened the Versions before that too, I only recently switched to Linux and Firefox), and someone else also complained it happened to him on Mint. It also happens to some Users on Windows 10 according to @OvermindDL1 who would be better qualified at explaining all this, and now he gets pinged for not wanting to tell it himself.
The Issue itself is that Firefox throws Errors either while logging in using oauth or when already being logged in with a session. It doesn’t happen when you are not logged in or attempt to log in, in some way shape or form.
I took Screenshots of it, while it happened during the Registration of this very Account I am writing this on.
It happens every single time I open a fresh Tab of any Discourse based Site in Firefox that I am logged into, then I need to F5 or hit “try again” and it works again just fine (well until I close the tab and need to open it later again ofcourse).
I could middleclick the bookmark to open the Forums on new tabs and all the tabs created through this will consistently throw that Error and prompt me to try again.
Funnily enough this even happens when the Internet is fully disconnected. Instead of “Failed to Reach Website” I get “Network Protocol Violation”, meaning the Cache is involved in this aswell.
Needless to say this is a very infuriating Issue.