In Firefox, how to avoid some security policies for local files?

[linuxmetel]

In Firefox, this add-on doesn't work for local files which loads local CSS files, you know. It's because of restriction on local file links and of the same-origin policy. The solution for Chrome is provided in Dark Reader Help, but not for Firefox.
So I tried to solve it, and I know how to avoid the restriction on local file links. Here's how:

1. open about:config.
2. add three values: capability.policy.policynames: localfilelinks, capability.policy.localfilelinks.checkloaduri.enabled: allAccess, and capability.policy.localfilelinks.sites: moz-extention://XXXX-XXXX-XXXX-XXXXXXXX/ (replace the URI of Dark Reader).

But I don't know how to avoid the same origin policy only for this addon (using security.fileuri.strict_origin_policy seems to be too risky for me). Is there any safe way to accomplish this?

And if there is a safe way to have Dark Reader work correctly for local files in Firefox, I think it should add into Dark Reader Help.