Server Site Events (SSE) connection abort with firefox

I wrote a little Chat using SSE for loading new messages. It is working fine with crome, safari and opera while firefox closes the connection after a few retrys. I'am using retry: 2000 (2s). Sometimes firefox is doing up to 10 events, sometimes only 1 or 2. I know from my serverstats, that more then 80% of my users are using firefox, so I need this working on firefox.

I'm using the latest version of firefox and I added a console.log() after every EventListener I'm using ('message', 'open', 'close').

Does anyone have an idea what to do?

Thanks a lot