(Not OP) I have no beef with Mozilla, love Firefox.

I once accidentally bookmarked some porn instead of opening it into a new tab. It happened on mobile, it happend fast, I barely noticed. It directly synced to all my devices. Even after removing the bookmark itself, Firefox kept suggesting this “last bookmarked page” on all my devices. Annoying if you alone know about it, disastrous if you don’t know who knows about it.

No, I just literally have no usecase for sync