I want to re-emphasize what Gijs says here. There are some architectural risks for Firefox that we can not properly address with these prefs existing. However, I am also very sympathetic to your user case. I've used the pref myself some times. But for your local developer setup, I recommend using a WebExtension to override CSP headers. I also know that other folks are successfully using security tools like ZAP Proxy or Burp Suite to remove security headers during testing.
Bug 1754301 Comment 10 Edit History
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I want to re-emphasize what Gijs says here. There are some architectural risks for Firefox that we can not properly address with these prefs existing. However, I am also very sympathetic to your use case. I've used the pref myself some times. But for your local developer setup, I recommend using a WebExtension to override CSP headers. I also know that other folks are successfully using security tools like ZAP Proxy or Burp Suite to remove security headers during testing.