The original blog post from Mozilla explicitly stated versions of Firefox without pdf.js were unaffected and a Mozilla employee confirmed disabling pdf.js would

The original blog post from Mozilla explicitly stated versions of Firefox without pdf.js were unaffected and a Mozilla employee confirmed disabling pdf.js would be enough to protect against the exploit. Mozilla was also extremely cagey about the nature of the exploit, its severity, and who might have been affected by it. If people were misled into thinking pdf.js was completely to blame, then that's Mozilla's fault.