Thanks Helen for your help. Cookies must be enabled although I've never seen one. May be its possible by software to recognise this situaton and print a differentiated message.
I installed resilio sync as server. But in firefox 54.0.1 the URL http://127.0.0.1:8888/gui/ unexpectedly shows an error message "sync does not support your web browser" (s. attachment). In addition I attached the firefox info in case of error. resilio sync as server in firefox.pdf