How does Firefox know my deadname?

I'm getting firefox set up, and when I tried to sync my phone, it said it was going to sync to [DEADNAME]'s desktop-[NUMBER]. Desktop-[NUMBER] really is the name of my desktop set in Windows settings, but I can't find any mention of my deadname from the Windows control panel. Firefox provides a mechanism to change the device name in Firefox, so I immediately did, but where did Firefox get this private information from? I installed Firefox long after I thought I removed all mentions of my deadname from my PC. I want to know where Firefox got this information so I can change it and prevent any future applications I install from using my deadname.