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

simple-modify-headers 1.8.1 can't add, change or remove parts of URL!
simple-modify-headers 1.8.1 can't use parts of URL in many places...
simple-modify-headers 1.8.1 can't replace or remove parts of URL in many places...

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.

This header works better than the older referrer control but it's missing an easy to add option to add site to be blocked. In Referrer Control it was a context menu to add it so did not even need the extension to even show on the toolbar. This one requires always showing on toolbar as I could not other way to start it and manually typing in to add a site to it.

Easily a 5 star if the convenience to add a site is much easier and quicker as well as not have to display more on an already crowded toolbar. Give and option menu in the Firefox add-ons page and give option not to delete all previous rules if you import a config in.

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!

Does nothing, headers are not changed.

Once it stopped working until I did not reinstall it.