With Ubuntu now using a snap version of Firefox there is a a problem with using the ThreeJS viewer with Firefox or Chrome in recent versions of Sage. Firefox can't access folders outside of a user's home folders nor even in hidden folders within the user's home folders. For Sage 9.6 it was possible to work around this by setting the environment variable DOT_SAGE to be a non-hidden folder, e.g., export DOT_SAGE="~/Sage/sage-9.6/dotsage" (on my PC Sage is installed in ~/Sage/sage-9.6). If you use a non snap browser this isn't necessary, but I prefer to use the default browser with Ubuntu.
With Sage 9.7 just setting DOT_SAGE no longer works, since the output from show(viewer="threejs") is now saved to /tmp/tmp<something>/<something>.html. The html file is fine, just not accessible by Firefox. I've looked in the the Sage documentation and haven't found a way to set the output path for the html files. (I may just have not found it, but any guidance would be appreciated.)