I posted this question on Stack Overflow, and the one response mentioned, "I guess from your mention of opendiff that you're using Mac OS. The recommended way to get sshfs on Mac OS used to be macfuse, but it looks as if that is no longer supported, so you might be better off with FuseX. (Only being a very infrequent Mac user, I can't personally vouch for either of those.)"
I followed the FuseX link and it said basically, "This is dead; you can subscribe to the Google group," and I followed that link, and it said basically, "This group is dead; do XYZ if you really want to be able to access it."
Is there any good, currently appropriate way to ssh mount an Ubuntu filesystem?