Login details such as that are stored within your Firefox installation, so that your data is as secure as your computer is. You can make that data even more secure by using a Master Password in Firefox, along with a password for your Windows Logon User Account. https://support.mozilla.org/en-US/kb/use-master-password-protect-stored-logins

The Yahoo hack which was in the new back in September and then again this week is different than storing your username & password for your email account inside of Firefox. Yahoo servers were hacked to get far more personal data than merely usernames & passwords for email accounts.

Unless you are using Mozilla or Firefox optional features which you haven't mentioned ...

Personally I would not rely on Firefox's Master Password for saving credentials I use for banking. The info is encrypted some of the time on the local computer, but If I Recall Correctly not to the highest standards maybe taking minutes to crack in a brute force attack.

The Sync data is encrypted end to end to high standards. I am not too sure I would trust Sync too much because

• I have seen mention of reliability issues
• I think it is not specifically designed to backup information. The backup is the data on your computer. So that relies on the Master Password at best.

For more info on Sync

• User Friendly summary www.mozilla.org/firefox/sync
• Our help documents (Link may change soon) [products/firefox/sync-and-save] /products/firefox/sync-and-save
• Mozilla Blog - Mozilla Cloud Services (2014) A Better Firefox Sync
• Technical details with explanatory graphics Firefox Accounts/Sync Protocol
• For those wishing to understand why Sync was set up & then revamped a couple of excellent Mozilla Blogs
  1. Brian Warner Pairing Problems
  2. Brian Warner The new Sync protocol

Thanks to both of you for replying to my question. I will change my email password (which is about due now anyway) and revert to entering it manually.