However I would LOVE to see an option to modify the URL string of a request :) If that is possible of course. Many thanks and good luck in your life!

Edit: Found Redirector extension for that :)

Only correct alternative to the great "Modify Headers" add-on for FF57.
But it could have a better IHM to manipulate the headers modification rules.
And it's very annoying that you can't modify an existing rule : you have to remove it and re-type it entirely.
Still looking for a better implementation for FF57...

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!