Pretty much every application that I use that has the ability to launch files seems to try to use Nautilus as the application to open every single file, regardless of file type. So, when I right-click on a file in Thunderbird's "Saved files" dialog and choose "open file," I get an Nautilus window popping up with an error message saying "Could not display '{name of file}.' The location is not a folder." This also happens in Firefox, Chromium, Zotero (as a Firefox extension and standalone), etc. etc. etc. etc.

I have no idea how to fix this, and haven't been able to find any other threads on the topic that wound up being helpful. Any help is much appreciated.