You should still keep all data you currently have in your computer if you reset the password of the Mozilla Account, only data stored on the Sync server is cleared. You can backup your personal data if you aren't confident or otherwise want to be safe. *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles If you reset the password for the Mozilla account because you do not remember this password, then that will remove all data stored on the Sync server because this password is used to encrypt your data locally before it is uploaded to the Sync server and only this password or a possible recovery key you created earlier can decrypt this data.