I have to say I was more than a little surprised when you got calibre involved with it. I assume it was necessitated by the built in Open With - seemed to happen about the same time.
The introduction of the 'Calamities on Toast feature' has only made things worse. Apart from Sigil putting its hand up to open my PDFs, Firefox put its hand up for CBZ's - when I took up its offer, it wanted to download 'file:///users/blah blah/Downloads/picturebook.cbz' to my -- Downloads folder
BR