Add a changelog for Firefox

[kneekoo]

This can be a copy/paste from the official website's release notes, but at least a link can be provided for convenience:

https://www.mozilla.org/firefox/{version}/releasenotes/

[github-actions]

This issue has been automatically marked as stale because it has not had
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If this issue is still relevant:

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[kneekoo]

This would still be useful.