Please note that it looks like adding /etc/firefox-esr/pureos.js is only masking the problem. The underlying implementation would still be built in the package and available in the source code. This has already been mentioned in the last paragraph of this comment: https://tracker.pureos.net/T902#16752

OK, if there's an interest from PureOS team to resolve these issues, I will do my best to file them separately.

I'm not sure if it was correct to initially set priority of this task to Freedom Issue, because it's possible the issue went unnoticed by team members.

I guess the best approach in resolving of the issue would be a collaboration between many parties interested in FSDG-compatible fork of Firefox.