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 article.

If you have a copy of your data on a mobile version of Firefox, please follow these instructions to connect it to Firefox Sync to copy your data.

No, you don't need to disconnect your old device from Sync before adding a new device.

Pleas see this Knowledge Base article and see if it helps you. https://support.mozilla.org/en-US/kb/firefox-sync-troubleshooting-and-tips

Questions: Do you still have the old device? If not, is there any chance the new owner might have cleared your personal data from that Firefox installation? Thus clearing your data from the Sync server.

Did you have any problem connecting that new laptop to your Firefox Account? Any chance that you requested a new Password for your Firefox Account?