Hi Thank you for your question about the Firefox Sync service. It appears from your question, that you are using Firefox Sync as a back up service. This is not what Sync is designed to do. The Firefox Sync service takes a copy of the data you wish to include and transfers it to a second device (typically a mobile device such as a tablet or a telephone) running a copy of Firefox. The storage in between all attached devices is both temporary and fragile and is not stable enough to be (and is not designed as) a reliable backup service. If you have a copy of your Firefox profile for desktop Firefox, you may be able to recover your bookmarks and other data. Please have a read of this [[Recover lost or missing Bookmarks| article]]. If you have a copy of your data on a mobile version of Firefox, please follow these [[How do I set up Sync on my computer?#w_connect-additional-devices-to-sync|instructions]] to connect it to Firefox Sync to copy your data.

Do you still have access to the old device? *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile If you use the Time Machine or other backup then try to restore file(s) from this backup. *https://support.apple.com/guide/mac-help/restore-files-mh11422/mac