fix https://github.com/sagemathinc/cocalc/issues/3771 -- basically ban Firefox 66.
fix https://github.com/sagemathinc/cocalc/issues/3771 -- basically ban Firefox 66.

tscholl2/smc,sagemathinc/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,sagemathinc/smc,DrXyzzy/smc