Remote access not working in firefox 57.0

I just discovered your great software and I am really happy with it. I ran into one hiccup though when setting up the access over the internet. I set up the external domain, did the port forwarding etc. But I I couldn’t connect to the web interface from the outside and the restore wouldn’t either. I just get a blank page and firefox is loading forever. So after checking all my settings I tried Chrome and later Edge and it would work there flawlessly. So I restarted firefox in save mode but that didn’t help either.
Acccesing the server in my local network with the local hostname is working fine though.
I guess this might be related to ff 57. Otherwise the forums would be packed with complaints about it. Or I just have something messed up here on my side. But I can’t imagine what that’d be…
Please help me out with some good ideas.