Would be great if there was an option to export and import headers so we can sync them between different Firefox installs.

Remove & modify both doesn't work
Once a rule added can't be edited
Unfortunately

Thanks for the addon but only add function works.

Tried contacting you using your website bug report but it always gives me an error after submitting.

It works as expected. Only missing feature is a better support to regex expressions.

i really needed multi User-Agent switcher like this.
but not work in FF 54.
any other condition required for use this?

Since RefControl is not compatible with firefox 57+ I use this addon to replace the Referer header. Works great!