That urging doesn't just come from Mozilla—it comes from the United States Department of Homeland Security's Cybersecurity and Infrastructure Security Agency (CISA).
The issue is this: Firefox versions for desktop older than the just-patched version contain a critical vulnerability that could allow an attacker to take control of a user's entire operating system—whether they use Windows or Mac. More alarming, the vulnerability is already being exploited in the wild, thus Homeland Security stepping in with the urgent plea for users to upgrade.
From CISA's warning:
Mozilla has released security updates to address a vulnerability in Firefox and Firefox ESR. An attacker could exploit this vulnerability to take control of an affected system. This vulnerability was detected in exploits in the wild.