AFAIK Firefox sync doesn't save anything in "the cloud". It's just synced from/to your other devices. The data is also encrypted using your Firefox account pass

AFAIK Firefox sync doesn't save anything in "the cloud". It's just synced from/to your other devices. The data is also encrypted using your Firefox account password, much like what cloud-based password managers like Bitwarden do. So I don't think Firefox Sync would be a bad way to get your data transferred.