Collaborative Editing / websockets not working when using Firefox

Dear team
While testing Collaborative Editing in the latest Confluence (6.0.0-m77), we noticed that when using firefox, collaborative editing does not work. The editor window does load, but at the bottom, right from the 'what did you change' field, it keeps saying 'Connecting...'
At the same time, in firefox developer console, websocket requests show up, but the server seems to return 400 errors. In other browsers like Edge, it works as expected.

This can be reproduced in Firefox on MacOS as well as Windows.

Best regards
Denis
bitvoodoo AG