The best would be to publish on issue on github : https://github.com/didierfred/SimpleModifyHeaders/issues.

And then we could echange and maybe I could solve your problem :

Could you give me the version of firefox you are using ? the version of the plugin ?
Could you configure an add header named "test" with value "test" and check with the following url http://eu.httpbin.org/headers if the header is modify ?
Could you send me your configuration file using the export fonction ?