That url implies that the following plugin is installed whatever you might think:
https://addons.mozilla.org/en-US/firefox/addon/lucifox/
It makes no difference what Application is set for EPUB files, Lucifox will intercept the file before it gets passed to that part of the FF code... in much the same way you cannot set a default Application to open HTML files.
If you say you have checked and it isn't installed, I don't know what to say... maybe try a fresh profile?
Is it possible Lucifox is being transparently loaded into Firefox by a program on your computer -- some programs hide the fact that they install addons to Firefox, and they don't show up up in about
:addons -- does Lucidor do that???