Sync isn't a backup system it sync the data between devices and if the original gets removed so does the sync on other devices. Only the data from the originals remains on the original locations. And if the account don't match sync will also remove unmatched data as well.

Are all connected devices desktop devices ?

This might be a problem with the services.sync.engine.bookmarks pref that controls whether bookmarks are synced that is still set to true on one of the connected devices. You can try to disconnect all devices and make sure you do not select Bookmarks when you reconnect.

• https://support.mozilla.org/en-US/kb/remove-synced-device-firefox-accounts
• https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox

You can see connected devices under "Devices & Apps" on the Firefox Account Settings page.

• https://accounts.firefox.com/signin
• https://accounts.firefox.com/settings

I'm not sure I understand what you're saying. The problem is, on the list of things that should be synced, I have Bookmarks checked. But (and this is something I just figured out) sometimes (but not always) when one of my computers is rebooted, when I next start Firefox, Bookmarks is no longer checked (and, of course, my bookmarks are no longer synced). It doesn't ever seem to happen if I just exit Firefox and then start it up again without rebooting the computer.

This is two desktops and one laptop, all running either Windows 10 or 11.

By the way, I also have Firefox Android on my phone and it is synced with the other computers, but it never seems to experience this problem.

Firefox stores the choices you make in "Settings -> Sync" in services.sync.engine.* prefs on the about:config page. If those settings get changed without you doing this then this means that its services.sync.engine.* control pref gets modified. For the bookmarks this is services.sync.engine.bookmarks which is true by default, so if syncing the bookmarks gets disabled then this could be done by security software that considers this a privacy issue. This could also propagate from one of the other devices connected to this Sync account.

• https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox

So...what do I do about it?!?