Such bad interface

Header rules can't be changed, you need to remove and add rule again. There is no export/import possibility.

It actually cannot modify the User-Agent header, if "Modify" selected. Had to click "Add" instead.

Can't edit saved settings

Works well, but for the lack of ability to edit a rule or even copy/paste the existing fields.
The fact that you can't edit a header to fix an issue makes this plugin a pain to use.
Only acceptable (compatible) headers tool add-on for Firefox Quantum.
But it could have a better UI to manipulate the headers modification rules.
Still looking for a better implementation for Firefox Quantum.

Works well, but minus 1-star for the lack of ability to edit a rule or even copy/paste the existing fields.

The fact that you can't edit a header to fix an issue makes this plugin a pain to use. And .... you can't even select it to copy and paste has made me remove the plugin.

It is too difficult to configure

Good friend for SW Engineer.
You should be need it.

Buggy as hell. The domain checkbox has no affect. Still affects the whole domain instead of just URL homepage even when unchecked. The modify and remove checkboxes also have no affect. The header don't get modified and remove doesn't work. The domain, modify and remove options need fixing.

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