The extension rewrite the headers based on a rules table. It's possible to:

• add , modify or delete header field for HTTP(S) request or response
• export the configuration in a file (json format)
• import the configuration from a file , it support the format of the Modifyheaders plugin
• choose urls on which the modifications applies

More information on https://github.com/didierfred/SimpleModifyHeaders/tree/v1.8.1 . The code is opensource under Mozilla Public License 2.0