There’s a perfectly plausible charitable interpretation offered by the reporters in comment 10.
They say that they filed this bug before they had devised their attack on the favicon cache; and so they reasonably asked, “why isn’t Firefox caching it like everyone else and as we believe everyone should?”—because as :mossop explains in comment 13, the spec suggests it should be cached, by remaining silent on the point.
Then, they developed the attack, and reported it to the affected browsers, which excluded Firefox. Certainly it was not great to leave it open without adding a comment saying “hey, don’t go ahead with fixing this yet, we developed a fingerprinting attack if it does get cached”, but it’s easy to understand this being overlooked. Also, as the reporters of the issue, they would receive any progress on the issue by email, so if you assume good faith, then they would have pumped the brakes if someone had actually gone ahead with implementing the initially-requested caching.
It’s possible that there was bad faith, but I find the good faith explanation entirely plausible—that there was a minor error of judgement only.