Allow changing preferences/keys instantly without restarting Firefox

[mishrashutosh]

Modifying a key or preference in `about:config` takes effect immediately, however doing the same with `user.js`, AutoConfig, or `policies.json` only takes effect after restarting a running instance of Firefox. There should be a way to programmatically change a Firefox preference and have it take effect immediately, just like it does in about:config!

[Jon]

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