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First one I found was blocking of eval()

https://searchfox.org/mozilla-central/source/dom/security/ns...

Essentially, it allows calling eval[1] and executing "privileged loads"[0] in some elevated contexts (the System Principal, which is responsible for the browser UI[2]).

Those are useful mitigations, but disabling them for some custom user-chrome is unlikely to have a meaningful impact on your browser's security.

[0] https://searchfox.org/mozilla-central/source/dom/security/ns...

[1] https://searchfox.org/mozilla-central/source/dom/security/ns...

[2]: https://blog.mozilla.org/attack-and-defense/2020/06/10/under...

As pointed out by sibling-comments, this seems to be in the context of making sandbox escapes harder.