The Thunderbird forum says there isn’t a problem, and I am wondering whether it is a Flatpak container problem. “Software” says that it is a “Fedora Flatpak”, so I assume that Fedora is responsible for Flatpak-ing it.
I wrote an email then clicked on “Attach” (same happens if I click “Add attachment”), I got an error message that stated that a completely unrelated directory did not exist. There was no way to get round this, so I could not add the attachment. I eventually worked out that the directory had been used for an attachment in a previous unrelated email and that directory had since been deleted.
I tried creating another directory of the same name and the attachment process worked.
In summary: When attaching, Thunderbird seems to default to the most recently used directory, if it no longer exists the attachment process fails and there is no way round it within Thunderbird.