If you are using Firefox Sync you can restore them.
Import and backup >
Restore and then pick a date to restore from. If you don’t see any bookmarks or restoration points there then you might be on a different profile and try to look in
about:profile if there are multiple profiles and try to run one of them.
I don’t know why you lost your bookmarks. I use Nightly (v55) which is basically the code they built yesterday and I never had such a serious bug in the past years since I started using it.
What addons do you have installed? Check