iOS/MacOS sync setup using QR code camera scan fails every time

Steps to reproduce

1) Delete Firefox (FF) iOS app and data from iPhone. Re-install fresh copy from App Store. 2) Log-in to FF account on Mac desktop app (v129.0, 64-bit). 3) Open "Account" menu in Mac desktop app and select "Sync settings" menu item. 4) Verify that "Bookmarks" are selected (checkmarked) for syncing in Mac desktop app. 5) Left-click "Connect another device" link at bottom of "Sync" settings page in Mac desktop app. 6) Left-click blue "Connect another device" button/link in Mac desktop app. 7) Highlight "I already have Firefox for mobile" button by left-clicking in Mac desktop app. 8) Left-click blue "Continue" button in Mac desktop app.

9) Open FF on iPhone. Follow instructions to make FF the default iPhone browser. 10) Return to iPhone FF app and select the highlighted "Sign In" button. 11) Select "Ready to Scan" button in iPhone app. 12) Select "Allow" button in iPhone QR reader app. 13) Point iPhone camera at QR code displayed in FF desktop app. 14) Select blue "Confirm pairing" button in iPhone FF app.

Expected behavior

13) QR code camera scan processes successfully. 14) Sync setup succeeds.

Actual behavior

1) Success 2) Success 3) "Sync" (settings) page is displayed 4) Success 5) "Connect another device" dialog is displayed with blue "Connect another device" link highlighted. 6) "Sync your Firefox experience" window is displayed. 7) "I already have Firefox for mobile" is highlighted. Blue "Continue" button also highlights. 8) QR code with instructions titled "Sync Firefox on your phone or tablet" is displayed on desktop. OK button also highlights.

9) Success 10) iPhone FF app displays "Sync and Save Data / Sign In with Your Camera" with "Ready to Scan" button highlighted. 11) iPhone QR reader states, "Firefox Would Like to Access the Camera". 12) iPhone camera is activated. 13) iPhone camera successfully captures desktop QR code. iPhone FF app displays "moz://a Confirm pairing for myemail@mydomain.com" with blue "Confirm pairing" button highlighted.

Desktop browser app fails immediately with "moz://a 500 error. Oh dear, something went wrong there. We've been notified and will get working on a fix."

14) iPhone displays "moz://a Approval now required from your other device." However, FF desktop app already failed with the "500 Error", which remains displayed.

15) After a few minutes, FF desktop app displays "Pairing not successful. WebSocket unexpectedly closed. The setup process was terminated. Please sign in with your email."

Note: I have attempted the QR code scan method many times with previous iOS/MacOS versions of the FF app. The QR method has NEVER worked. It has always failed with the "500 Error" in the desktop app.