The Sync Key is used to encrypt and decrypt your data, as your data is uploaded to and downloaded from the Sync server. Even of you could "access" your data it would be useless without the Sync Key.

Did you use that same Sync account with any other devices?
If so, look in the Password Manager > Saved Passwords to see your Sync Key.

If not, you learned the hard way about how important the Sync Key is - you should have written it down.

See also Firefox Sync troubleshooting and tips (I've forgotten my Secret Phrase, what can I do?)

I'm sorry but "you should have written it down" simply isn't a good solution. Having the keys at all is a bad idea in my opinion. The keys should be stored in your account and if you authenticate with a password, it should use that key. If you can't keep your password private or secure, that's your problem. The problem with the keys is, you can't delete your account and "start over" if you so wish. What happens if your machine locks up after you sign up and you have no time to write down your key?

You can request a new password, but can't be reminded of your key? This is nonsense. I have had no such headaches with XMarks and I'm trying to find a reason to keep using firefox sync with such an obviously user-unfriendly approach. We're humans, not machines.

1. If you don't have time to write down your Sync Key and your machine locks up after you sign up, simply create another new account.

2. No you can't be reminded of your key, it is stored on each PC that is setup with Sync; it isn't stored by Mozilla on the Sync server for your security - only you can read your data, not even Mozilla personnel can read your data. Xmarks is setup so that they have access to your bookmarks, to do with as they wish within the realm of their privacy statement.

"Simply create another new account." Really? Should I create a new E-Mail address then too, since the address I used is already in the system? Come on, you're just making excuses for something that's flawed.

People shouldn't be writing down keys. That's about as insecure as it gets. I stored my key in a TrueCrypt volume. Most people don't know how to do this so will likely store it in email, on a piece of paper etc. This defeats the purpose of security does it not? But what if my TrueCrypt volume is corrupted? Should I buy a safe? Sheesh!

And FYI, it's entirely possible for the key to be stored on the Sync Server in such a fashion that it would be unreadable by anyone who didn't have your password. The key can be encrypted using a hash that could be randomly generated in a step-by-step process taking random input from the user. Then, only the user's password would unlock the data. Changing one's password does NOT have to be supported, but even that could be accomplished, it would just take overhead because the data would have to be decrypted first, then re-encrypted. This isn't rocket science.

Piece of junk add on if you can't retrieve your key - who is going to write down that LONG key that is given? I'll stick with Last Pass - it's WAY better and works on BOTH Chrome and Firefox.

Please do not tell people to write down security keys - any software worth its weight has a retrieval option or a reset option. Fix the program so it works like every other program works.

To retrieve the sync key, in the Options/Preferences dialog, select the Sync panel. Click the Manage Account button and then the My Sync Key link.

Blaming the user for not writing down the sync key because they "should have known better" is stupid. Sure, the Sync Servers don't store things in plain text because this could be bad. Sure, the sync servers don't store your sync password because this is also bad. You know what is also bad? Storing passwords in the first place!

For me, this isn't about Firefox being security cautious by using AES or whatever other symmetric key algorithm, it's about Firefox being user oblivious. I am cautious, and I am not an idiot. If I have the choice, I don't let websites store information that I think they shouldn't have. Because of this, I would never store my passwords using Firefox Sync. I would never store my history either. I would, however, store my bookmarks. These are just URLs. They aren't secrets! Store these in plain text and save me the burden of the unneeded security overhead. I should be able to opt out of the superfluous security. I'll even offer a compromise: the user can only opt out if they don't intend on storing passwords.

And to clarify, I used "would" earlier instead of "will" because I don't plan on using Sync until they fix this. Yes, fix this; problems aren't changed, they are fixed.

I can't get to the "manage account" button because I cant log in without the stupid key! Now what? I made a lot of changes to my bookmarks and I can't access any of it now because I need a key that I didn't even know I had! How can I start over and retrieve my sync data without this key?