History and bookmarks are always stored and synced in firefox if you have enabled this. How did they vanish, did you install some extension or something happened? Try to go the history section or the top left icon if you are on this version, it holds your history.

I have this problem also. Then Firefox wouldn't open at all. I reinstalled it, but even when logged in, no bookmarks. Oddly, passwords ARE still there. This is on a Windows 10 desktop. My bookmarks are all there on my Android phone, but I want to make sure restoration is from there, and syncing won't erase the phone in favor of the blank desktop.