Can't change existing header after adding it.
Even when you add a new header, it is not reliably added. I wish the old ModHeader extension was still available.

It works, but the UI for enabling it very badly designed. The UI element just toggles between blue and white, with no indication which ones is on and which one is off.

Once headers is configured, its value cannot be changed. Too bad when using Authentication that changes often. Uninstalled.

Such bad interface

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

No regex, no subdomains filter. It's either exact site address match or star (everything)

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.

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...