Computer Configuration > Administrative Templates > Firefox
Both
HKLM\Software\Policies\Mozilla\Firefox\LegacySameSiteCookieBehaviorEnabledForDomainList
If this policy is enabled, Firefox will revert to the legacy behavior of SameSite for the specified domains. This means that cookies that don't explicitly specify a SameSite attribute are treated as if they were SameSite=None. If this policy is disabled or not configured, Firefox will enforce SameSite=lax for all domains.