Sharing from Firefox Android creates a duplicate note

### Expected
Sharing a page from Firefox Android and pressing to back button should take user back to browser.

### Observed
When pressing to back button for the first time it creates a duplicate note, pressing it for second time it takes user back to browser.

### Reproduced
1. Go to Firefox Android and open a URL.
2. Share it to Simplenote.
3. Press back button from top left of app or navigation bar.

Device Maker: Huawei
Device Model: P20 Lite
Android Version: 9
Simplenote Version: 2.23 (148)
Firefox Android Version: 97.3.0

Start by reproducing the Firefox Android share flow on the reported Android setup, then trace the app's share handling and back-navigation entry points. Done means the first back action returns to Firefox without creating a duplicate note, while the existing share behavior remains intact.

Written by the indexing model from the issue text.