The add-on requires permission to access only relevant Github pages, but it's only evaluating the URLs and determining whether or not to redirect to the same URL with "w=1" appended. It is not reading any content.

Full source code is available here:
https://github.com/maksimovic/github-whitespace-disabler

Install

• License: MIT License
• Weekly downloads: 2
• Average daily users: 36
• Rating: 4/5 of 1 ratings
• Created: 2017-06-30 07:24:19
• Last updated: 2022-02-19 20:56:22
• Homepage:
• Support site and email
• Orig: https://addons.mozilla.org/en-US/firefox/addon/github-whitespace-disabler/
• API: maksimovic@outlook.com