It's under the Sync preference pane in Firefox Preferences.

Thats the problem. This page does not exist. If i click on this tab it only has 2 buttons, Create new account or connect device. I am looking for exactly this setting that you described! I just cant find that screen. All the tutorials show it but i dont know how to get there. I am not stupid, i am a programmer/webdesigner.

Which version of Firefox do you have installed? Did you try running with a new profile (firefox.exe -P)? Perhaps you have an older version of the Sync extension running and it's conflicting somehow.

No, no sync extension installed. I will just wait i guess it will solve its self somehow trough updates in the future. I dont need it that bad i just thought it would be a nice feature, just wanted to try it with my android phone and iphone.

I have the latest firefox installed.

But thanks for your help.

Here is a screenshot...

Right. The problem is that you need to properly set up Sync.

Click the button there, 'Set up Firefox Sync'. If you don't have an account select 'Create a new Account', otherwise at the bottom, 'I already have a Sync account' -> 'I don't have a device with me' -> complete all details -> continue -- and then you will see the 'Add a device' option in the Sync preferences pane.

The setup is complicated, convoluted and not ideal, and it is being worked on, but that should resolve your issue.

I agree that this is a lesson in futility.

The box with 3 slots shown in the tutorial will not accept the key code.

I already have a sync account and have reset the sync key 3 times and NONE of the codes work to get me to the next page in the sync process.