The thread from which the two statements originate is locked, so that I can't ask @mrlightman there which concrete Firefox and Opera versions the import was successful with.

The second member I quoted has left the forum in the meantime. In June he had told a Mac owner who was complaining about the same problem:

[The import from Firefox] worked fine for me. If you want to import your passwords from Firefox into Opera, but have set a master password in Firefox, delete this in Firefox, close Firefox and then do the import with Opera. [...]

The OP mentioned MACOS 10.14.5, Firefox 67.0.1 and Opera 60.0.3255.160. There is no feedback from him as to whether he succeeded in importing his passwords based on this advice.

Again, I beg your pardon, if that doesn't work.