I don't know how to set this from any setting tool. On my system there is the file ".local/share/applications/mimeapps.list" containing the following lines
[Default Applications] text/html=firefox.desktop text/xml=firefox.desktop x-scheme-handler/mailto=icedove.desktop x-scheme-handler/http=firefox.desktop x-scheme-handler/https=firefox.desktop x-scheme-handler/ftp=firefox.desktop Regards, jvp.
