MOZ_FALLTHROUGH is an annotation to suppress clang's -Wimplicit-fallthrough warnings about switch cases that fall through without a break or return statement. MOZ_FALLTHROUGH is only needed on cases that have code: switch (foo) { case 1: // These cases have no code. No fallthrough annotations are needed. case 2: case 3: foo = 4; // This case has code, so a fallthrough annotation is needed: MOZ_FALLTHROUGH; default: return foo; } widget/cocoa/nsChildView.mm:1664:5 [-Wimplicit-fallthrough] unannotated fall-through between switch labels widget/cocoa/nsChildView.mm:1761:5 [-Wimplicit-fallthrough] unannotated fall-through between switch labels widget/cocoa/nsNativeThemeCocoa.mm:3700:5 [-Wimplicit-fallthrough] unannotated fall-through between switch labels