Good work. There are some hardening options that you may be able to glean from ArkenFox [1] and Betterfox [2]. Another addon to consider listing is CSS Exfil protection [3a] CSS Exfil Test Site [3b].

[1] - https://github.com/arkenfox/user.js

[2] - https://github.com/yokoffing/Betterfox

[3a] - https://addons.mozilla.org/en-US/firefox/addon/css-exfil-pro...

[3b] - https://www.mike-gualtieri.com/css-exfil-vulnerability-teste...