Like you say, Adobe's Flash sandbox (aka "Protected Mode", based on Chrome's sandbox library) only supports 32-bit Windows. Mozilla wrote its own plugin sandbox

Like you say, Adobe's Flash sandbox (aka "Protected Mode", based on Chrome's sandbox library) only supports 32-bit Windows. Mozilla wrote its own plugin sandbox for 64-bit Windows because we didn't want to Firefox users to lose sandboxing just because they switched from 32-bit to 64-bit. Adobe's and Mozilla's sandboxes don't use all the same mitigations and some Flash content is currently broken in 64-bit Firefox. Here is the Firefox bug tracking the 64-bit sandbox work: https://bugzilla.mozilla.org/show_bug.cgi?id=1165891 https://bugzilla.mozilla.org/show_bug.cgi?id=1165891