Resetting the sync password, or changing its e-mail or password, will remove all data from the servers. The username and password are used to create the encryption.

This is a security feature. https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information

• I've lost my Firefox Sync account information - What to do
• I’ve forgotten, or don't have access to, my Firefox Account email address

Note: Only the data on the servers is cleared. The profile data is not changed or removed.

You shouldn't lose bookmarks on connected devices and once you reconnect all devices using the new password then you should see the bookmarks from other devices.

If you lost bookmarks then on a desktop device you should be able to use an automatically created backup in the bookmarkbackups folder.

• https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them
• https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks