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.
In the Firefox Sync Account section, click on Add a Device and the Add a Device window will open.
Enter the code in the Add a Device window as it's displayed on your second device. Then click Next
Continue.
See Where can I find the code to add a device to Firefox Sync? for more information
When the code is accepted you will see a Device Connected message in Firefox and your device will immediately begin syncing.

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