Where does firefox get the “default” applications for opening files from?

By default, the Firefox (33.0) on my FreeBSD 11.0-CURRENT has the default application for opening PDF files set to Inkscape. Firefox does remember my previous choice, *evince*, in the “What should Firefox do with this file?” dialog, so until recently I was just confused where this configuration came from, but mostly ignored it because it did barely concern me. I have however recently started using Zotero as my literature database. Zotero runs as a firefox plugin, and ignores the choices made in my Firefox preferences to open PDF files with evince or ask me what to do with them, and just opens them using Inkscape. This made me try to follow up on this particular configuration item of Firefox, but I could not find where that default is set. I could not find the string Inkscape (or inkscape) in any Firefox-related file in my home directory.