https://support.mozilla.org/en-US/kb/how-do-i-set-up-firefox-sync

Tip. If you happen to use Firefox Sync, and opt to use it for bookmarks i recommend you backup your Firefox profile or at least your bookmarks manaully prior to using Sync for the first time, and then on a regular basis. Maybe every two weeks.

Do NOT become over reliant on Sync. Sync occasionally has problems, especially if you have a slow connection or if you change a lot of bookmarks. It is capable of very badly messing-up your bookmarks listings.

Also remember if used with an Android phone it will put all of your computer bookmarks on to the phone. There is no choice to just sync unidirectionally from the phone to the PC. The only action is to Merge bookmarks.

I don't care about bookmarks. I'm just asking what this sync thing is What is Sync?

Sync synchronises Firefox between devices.

• How do I set up Sync on my computer?

  Firefox Sync lets you share all of your data and preferences (such as your bookmarks, history, passwords, open tabs and installed add-ons) across all your devices. This article shows you how to set up Firefox Sync.

• Please also see https://www.mozilla.org/firefox/sync/