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
   Repeat the last two steps on the other Android device.

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:

Profile Manager - Create, remove or switch Firefox profiles

No computer. Someone suggested an app but it also required a computer.

No computer. someone suggested an app that cost a dollar and still used a computer

How about some help here? NO COMPUTER