[WebExtensions] Document Firefox and Safari support for DNR header "append" modification #46233

MDN URL

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/declarativeNetRequest/ModifyHeaderInfo

What specific section or headline is this issue about?

N/A

What information was incorrect, unhelpful, or incomplete?

Missing information about Firefox and Safari support.

What did you expect to see?

List for Safari and Firefox.

Do you have any supporting links, references, or citations?

No response

Do you have anything more you want to share?

No response