I am logged into the same Firefox account on both my phone and desktop. Sync is enabled and bookmarks are ticked. My desktop bookmarks do not show up in my phones bookmarks, and nothing I bookmark on my phone shows up in my desktop bookmarks. I remember there used to be a mobile bookmarks folder where these were stored, but that has disappeared from both my phone and desktop bookmark manager. I have a lot of open tabs on my phone which I need to bookmark and organise, so I can't reinstall to test if that fixes it without losing them all. I can see my phones open tabs on desktop through synced open tabs, so I'm definitely connected to the right account and clearly some stuff is syncing, just not bookmarks.