Doesn't work on old reddit

**Describe the bug**
I use old reddit by default, and harper doesn't activate when I type in either the thread reply text box or a comment reply. The only way I can make it work is to use Reddit Enhancement Suite's big editor, or to view the page in new reddit and type in the new non-markdown editor.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to any old.reddit.com post
2. Click on the reply box
3. Try to type something and intentionally make an error
4. See that nothing happens

**Expected behavior**
Harper to highlight spelling and grammar issues

**Screenshots**

**Platform**
Firefox

**Additional context**
I tried disabling other add-ons and double checked to make sure harper is enabled on that page and that I have the needed permissions enabled, but nothing I've tried has made it work.

The issue names no source files or tests. Reproduce the failure in Firefox on an old.reddit.com post, focusing on the thread reply and comment reply boxes, then trace how Harper activates on editable fields. Done means spelling and grammar issues are highlighted in both old Reddit reply boxes, while the existing working editors remain functional.

Written by the indexing model from the issue text.