There used to be an option in the Sync "Firefox Accounts" (Prefs > Sync > Manage devices) to rename devices. It seems it has been removed (probably recently, my bet in the past couple months). This is not a big issue by any mean, but I do find a device name like "hostname" significantly easier to parse than "username's Firefox on hostname". If the device renaming feature has been relocated, I'd be glad to know where it is now. If the feature has been removed entirely, and this is not the best place for this sort of feedback, I'd be happy to try and post in a more appropriate place I would be pointed to.