Adjustable spans for text don't show up in Firefox

**Describe the bug**
The adjustable spans implemented [here](https://github.com/HumanSignal/label-studio/pull/7309) don't show up in Firefox 153.0.3 (aarch64) for Mac.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to the [playground](https://labelstud.io/playground/?config=%3CView%3E%0A%20%20%3CText%20name%3D%22text-1%22%20value%3D%22%24text%22%20granularity%3D%22word%22%20highlightColor%3D%22%23ff0000%22%20%2F%3E%0A%20%20%3CLabels%20name%3D%22ner%22%20toName%3D%22text-1%22%3E%0A%20%20%20%20%3CLabel%20value%3D%22Person%22%20%2F%3E%0A%20%20%20%20%3CLabel%20value%3D%22Location%22%20%2F%3E%0A%20%20%3C%2FLabels%3E%0A%3C%2FView%3E) in Chrome and Firefox
2. Click on any of the labels
3. Select text
4. See the blue dots for text span appear in Chrome but not Firefox

**Expected behavior**
The text span handles should appear

**Screenshots**

**Environment (please complete the following information):**
- OS: Mac OS 26.6.1
- Label Studio Version: Playground and also local:
```json
{
"release": "1.21.0",
"label-studio-os-package": {
"version": "1.21.0",
"short_version": "1.21",
"latest_version_from_pypi": "1.23.0",
"latest_version_upload_time": "2026-03-13T08:20:25",
"current_version_is_outdated": true
},

"label-studio-os-backend": {
"message": "Add code to format PEFT notes to be annotated",
"commit": "7b8667db953297e321a81ac158ea35768c41cbf5",
"date": "2026/05/19 11:59:53",
"branch": "main",
"version": "7b8667d+dirty"
},

"label-studio-frontend": {
"message": "feat: BROS-353: Databricks storage integration (#8245) (#8542)",
"commit": "5c988322ab98dc71c4dbc3a2e66159d588038888",
"date": "2025-09-26T14:27:26.000Z",
"branch": "develop"
},

"dm2": {
"message": "feat: BROS-353: Databricks storage integration (#8245) (#8542)",
"commit": "5c988322ab98dc71c4dbc3a2e66159d588038888",
"date": "2025-09-26T14:27:26.000Z",
"branch": "develop"
},

"label-studio-converter": {
"version": "2.0.4"
},

"edition": "Community"
}
```

**Additional context**
Add any other context about the problem here.

Reproduce the issue in the linked playground using Chrome and Firefox, then inspect the adjustable-span implementation introduced by PR #7309. Compare the text-selection behavior between browsers and verify that the blue text-span handles appear in Firefox as they do in Chrome.

Written by the indexing model from the issue text.