Beware - headers in Developer Tools are raw. If you see them unmodified, it does not mean the extension does not work..

good, simple and works

f- you, //i.redd.it/

Super basic, but it works well, unlike some other extensions that do the same thing.

The UI is modern / clean and relatively easy to understand unlike the bulk of other header modifiers I've tried. It's open source and free (there's no stupid paid for features) and there's no clause that says it will phone home. Last update was relatively recent in 2022. I may keep this one.

However I'm not having success with disabling CSP for testing purposes. I tried Action:Delete => Header:content-security-policy => Applied to both response/request and Action: Modifiy: content-security-policy | field value : (empty) | on response without success.

Anyways. After trying it for a little bit. Some features I would like to see if indication if the addon is running on the opened webpage, maybe an option to report that information in console and which rules are being applied, and a comparison of the original vs modified header. I hope the developer continues to improve upon this add-on.

Works fine on Firefox 78.0.1 (64-bit).
Note: As Avinash-Bhat wrote in the SimpleModifyHeaders GitHub issue page, URL paths should not contain port. "http://localhost:1234" will match with "http://localhost/*".

It just .... simply works!

Once it stopped working until I did not reinstall it.

The only thing I miss is ability to set multiple URL pattern.