Data in Firefox Sync is end-to-end encrypted, while data synced to Chrome is subject to Google's privacy policy.

"The security model of Firefox Sync ensures that your sync data is encrypted before it ever leaves your machine, and that the password to unlock this encryption is never transmitted to the server. Not even Mozilla can access your sync data, so while we transmit it all over TLS for additional security, we do not depend on the confidentiality of TLS to keep your data safe."

https://medium.com/mozilla-tech/how-firefox-sync-keeps-your-...

"When you sync Chrome with your Google Account, we use your browsing data to improve and personalize your experience within Chrome. You can also personalize your experience on other Google products, by allowing your Chrome history to be included in your Google Web & App Activity."

https://www.google.com/chrome/privacy/

There is also the option to e2e encrypt your chrome data.

Because collecting user information is Google’s main business.

Have Google said they can't see your data when syncing?