Restarts instead of shutting down

Ever since I updated to Catalina (I have an iMac, late 2013), it seems like 2/3 of the time I shut down the computer, it restarts instead. When I log in, it says that the computer was shut down "due to an error." Any idea what is going on? I kind of suspect that it's due to an issue between Catalina and Firefox (which I have been using instead of Safari). Firefox is also updated.

Any ideas what's going on? And/or how to fix this issue?