1.Open Firefox and click on the three horizontal lines in the upper-right corner to open the menu. 2.Click on "Logins and Passwords." 3.Click on the three horizontal lines in the upper-right corner again and select "Sync Options." 4.Click on "Manage Synced Devices." You should see a list of devices that are currently synced with your Firefox account. If the device you are using is not listed, it may not be synced with your account. If it is listed, click on it to see what data is synced. You can try turning off sync for everything except "Logins and Passwords" and see if your missing credentials reappear. If your missing credentials do not reappear, you can try contacting Firefox support for further assistance.