Thanks for the answer. I am using Fedora 29, firefox-66.0-9.fc29; my settings are also
4,4,1 and changing as suggested in the link do not work. Other people in my University can
get in with firefox using gentoo or ubuntu.
Something strange just happen. I have an account in an ubuntu machine. I made ssh -CX4 to
this machine and open firefox; same issue. I tried again with x2go, no problem. Maybe it
helps.