Harper Firefox Extension not working on iCloud Web (Pages, Keynote, Numbers)

**Describe the bug**
Harper doesn't work on iCloud web apps on Firefox.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to 'icloud.com/pages'
2. Click on Enable on this website
3. Optionally disable spellcheck in Preferences
3. Doesn't work

**Expected behavior**
I expect it to work on iCloud web apps.

**Screenshots**

**Platform**
I am running the Harper extension on Firefox. My device is Fedora Linux.

**Additional context**
None

The report names the Firefox extension and iCloud Pages at icloud.com/pages; first reproduce the failure there with the extension enabled and spellcheck optionally disabled. Done means Harper works in the iCloud web apps on Firefox, with the expected behavior confirmed for Pages, Keynote, and Numbers.

Written by the indexing model from the issue text.