Firefox extension autofill broken — InputJsonDeserializationFailed

Since updating to 1Password 8.12.5 on Linux, the autofill feature no longer works in Firefox-based browsers (Firefox and Zen Browser). The same extension version (8.12.4.46) works correctly in Chromium. Copying passwords from the extension popup works fine — only the fill/autofill operation is broken.

Environment:

• OS: Arch Linux (kernel 6.19.6)
• 1Password app: 8.12.5 (also tested with beta 8.12.6-35 — same issue)
• 1Password extension: 8.12.4.46 (latest on AMO)
• Firefox: 148.0 (also tested with 147.0.4 — same issue)
• Chromium: autofill works with the same extension version and app

Steps to reproduce:

1. Open any login page in Firefox
2. Focus on a username/password input field
3. The 1Password inline menu appears and shows matching logins — this works
4. Click on a login to fill — nothing happens

Filling via the toolbar popup and keyboard shortcut also does nothing. Copying a password works fine.

Error from about:debugging → 1Password → Inspect:

Uncaught (in promise) Error: ⛔ Core error code: InputJsonDeserializationFailed

  sendCoreMessage background.js:108

  nextFill background.js:108

What I've tested:

The Firefox extension's fill request (nextFill → sendCoreMessage) is rejected by BrowserSupport with InputJsonDeserializationFailed. The same extension version works in Chromium, so the issue is in the Firefox-specific fill payload. This is a regression from the 8.12.2 → 8.12.5 app update (downgrading is blocked by database migration).