There no way that Mozilla or others can access data you uploaded to the Sync server as all data is encrypted locally using a key derived from the password of the Sync account and only this password can be used to decrypt this data. That is why it is important that you do not lose/forget this password or the recovery key if you generated one.

• https://support.mozilla.org/en-US/kb/reset-your-firefox-account-password-recovery-keys

If you use 2FA for more security then you also need to backup its recovery codes.

• https://support.mozilla.org/en-US/kb/secure-firefox-account-two-step-authentication
• https://support.mozilla.org/en-US/kb/what-if-im-locked-out-two-step-authentication

Thanks for your answer! But regardless if they were able to use the data, did security authorities had access to mozilla sync data in the past?

Sync has always worked this way, so they would have to ask you to give the password or inspect your computer directly assuming you use a password that is strong enough.

• https://wiki.mozilla.org/Labs/Weave/Crypto (2010)
• https://mozilla-services.readthedocs.io/en/latest/index.html