Harper does not work on Firefox ESR (140.9)

**Describe the bug**
A clear and concise description of what the bug is.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to 'Settings'
2. Click on 'Extension and Themes'
3. Scroll down to 'Harper'
4. See error

**Expected behavior**
it says that Harper is incompatible with Firefox ESR

**Screenshots**

**Platform**
Firefox ESR for Linux

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

Start by reproducing the incompatibility in Firefox ESR 140.9 for Linux using the Settings > Extension and Themes path described in the issue. Trace the Firefox extension compatibility configuration and determine what needs checking; done means Harper is recognized as compatible and can be used on that ESR version.

Written by the indexing model from the issue text.