In the Menu bar at the top of the screen, click Firefox and then select Preferences or Settings, depending on your macOS version.Click the menu button and select Settings. . In the OptionsPreferences window, click on the Sync tab.
Click Set Up Firefox Sync.
Click Create a New Account.
Enter your email address, create a password and confirm it. Then check "I agree to the Terms of Service and Privacy Policy" and click NextContinue.
This is your Sync KeyRecovery Key. Now is a good time to save or print a copy of your key and store it somewhere safe. When you are done with this step, click NextContinue.

The Sync KeyRecovery Key is used to encrypt your data before it's sent to the Mozilla servers. Think of it as a key that locks your information in a vault that only you can open. This means that neither Mozilla nor anyone else can read your information without having your Sync KeyRecovery Key to unlock it.
Confirm that you are not a robot by entering the text from the image. Then click NextContinue.
You will see a Setup Complete! message. Click FinishDone to close the setup window and take you back to the Sync panel.
You do not need to do anything else. Firefox Sync will automatically sync your data in the background from here on.
This first sync should happen quickly but may take more than 30 minutes depending on how much information needs to be synced.

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