There isn't an official way to do this.

Here's a hack which could work but I've never tried it and it's difficult to do:

1. Borrow a desktop or laptop with Firefox
2. Create a new profile
3. Set up a sync account
4. Write down your username, password and recovery key <- super important. See, Where can I find my Recovery Key?
5. Delete that profile you just created on the borrowed computer.
6. On one of your Android devices, set up sync and tap the "Advanced Setup" link
7. Enter your username, password and recovery key
8. Repeat the last two steps on the other Android device.

Verdi's process should work.

Fundamentally, the problem is that a computer is needed to complete the set-up. This is defined by the design of the feature.

The other option is to pair the 2 devices against a temporary computer, and then delete the profile from the borrowed computer.

If you want to learn how to create and delete profiles, check this article:

Use the Profile Manager to create and remove Firefox profiles

I hope it helps. Ibai