I can see it as well.
I don't understand it either.
I use facebook container on the desktop.
Perhaps it is on Android or iPhone?
I would also like to know what data Firefox shared with Facebook about me.
If you follow the trail of links (or directly go to https://www.facebook.com/dyi), you can request a detailed report. I have my doubts about on how complete this is, but in my case, the only thing I saw from Firefox were ACTIVATE_APP events; based on the timestamps almost definitely all from my phone.
Maybe it detects you using facebook container and infers you use firefox from that? Mine doesn't show anything firefox related, using ublock origin everywhere.
I can see it as well. I don't understand it either. I use facebook container on the desktop. Perhaps it is on Android or iPhone? I would also like to know what data Firefox shared with Facebook about me.
https://i.imgur.com/w8h9HeS.png