Do you have your data on another device? Sync isn't meant as a backup, but is meant to sync data between other connected devices. You should always backup your personal data yourself.

Nope. I am using the same device.

Did you already log into your Firefox account?

A trick I learned about Sync is that it will see a different version of Firefox as a different device on the same computer. You can sync your data between the two versions like a backup. The screenshot shows that my data is on different version and devices. When 1 browser is missing data, I can open another or go to a different device and send the data.

I already logged in to my firefox account. How can I do what you've done? Also a note, when I uninstalled firefox I didn't delete the residual folders and when I reinstalled it, it prompted me to reinstall instead of a fresh install but I also used Refresh Firefox options to remove the add-ons.

To test, download a different version of Firefox and make sure to install in a different directory. Log into it make sure sync is setup and the options are checked. I will assume that you logged into the browser with nothing and that is what will sync. This little test will verify what sync has.

https://www.mozilla.org/en-US/firefox/all/#product-desktop-beta