When launching Conkeror with FF48 nightly using firefox -app, Firefox will be opened instead of Conkeror.

This happens with these sets of arguments
~/src/firefox-48a1/firefox -app ~/src/conkeror/application.ini
~/src/firefox-48a1/firefox -app ~/src/conkeror/application.ini -new-instance
~/src/firefox-48a1/firefox -app ~/src/conkeror/application.ini -P

The only currently known workaround is to pass the profile name.

~/src/firefox-48a1/firefox -app ~/src/conkeror/application.ini -P profilename

I have created a FF issue:
https://bugzilla.mozilla.org/show_bug.cgi?id=1271574