Are you using https://addons.mozilla.org/en-US/firefox/addon/multi-account-containers/? I don't believe we natively support opening links in specific countainers.
Bug 1999477 Comment 1 Edit History
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Are you using https://addons.mozilla.org/en-US/firefox/addon/multi-account-containers/? I don't believe we natively support opening links in specific containers.