diff --git a/pkgs/development/compilers/llvm/13/clang/default.nix b/pkgs/development/compilers/llvm/13/clang/default.nix
index a775af3031b7..ab826fa31526 100644
--- a/pkgs/development/compilers/llvm/13/clang/default.nix
+++ b/pkgs/development/compilers/llvm/13/clang/default.nix
@@ -37,6 +37,11 @@ let
       ./purity.patch
       # https://reviews.llvm.org/D51899
       ./gnu-install-dirs.patch
+      # Revert of https://reviews.llvm.org/D100879
+      # The malloc alignment assumption is incorrect for jemalloc and causes
+      # mis-compilation in firefox.
+      # See: https://bugzilla.mozilla.org/show_bug.cgi?id=1741454
+      ./revert-malloc-alignment-assumption.patch
       (substituteAll {
         src = ../../clang-11-12-LLVMgold-path.patch;
         libllvmLibdir = "${libllvm.lib}/lib";
diff --git a/pkgs/development/compilers/llvm/13/clang/revert-malloc-alignment-assumption.patch b/pkgs/development/compilers/llvm/13/clang/revert-malloc-alignment-assumption.patch
new file mode 100644
index 000000000000..f8e55ff7f897
--- /dev/null
+++ b/pkgs/development/compilers/llvm/13/clang/revert-malloc-alignment-assumption.patch
@@ -0,0 +1,78 @@
+--- b/include/clang/Basic/TargetInfo.h
++++ a/include/clang/Basic/TargetInfo.h
+@@ -612,8 +612,8 @@
+   }
+ 
+   /// Return the largest alignment for which a suitably-sized allocation with
++  /// '::operator new(size_t)' is guaranteed to produce a correctly-aligned
++  /// pointer.
+-  /// '::operator new(size_t)' or 'malloc' is guaranteed to produce a
+-  /// correctly-aligned pointer.
+   unsigned getNewAlign() const {
+     return NewAlign ? NewAlign : std::max(LongDoubleAlign, LongLongAlign);
+   }
+--- b/lib/CodeGen/CGCall.cpp
++++ a/lib/CodeGen/CGCall.cpp
+@@ -2048,24 +2048,6 @@
+       // allows it to work on indirect virtual function calls.
+       if (AttrOnCallSite && TargetDecl->hasAttr<NoMergeAttr>())
+         FuncAttrs.addAttribute(llvm::Attribute::NoMerge);
+-
+-      // Add known guaranteed alignment for allocation functions.
+-      if (unsigned BuiltinID = Fn->getBuiltinID()) {
+-        switch (BuiltinID) {
+-        case Builtin::BIaligned_alloc:
+-        case Builtin::BIcalloc:
+-        case Builtin::BImalloc:
+-        case Builtin::BImemalign:
+-        case Builtin::BIrealloc:
+-        case Builtin::BIstrdup:
+-        case Builtin::BIstrndup:
+-          RetAttrs.addAlignmentAttr(Context.getTargetInfo().getNewAlign() /
+-                                    Context.getTargetInfo().getCharWidth());
+-          break;
+-        default:
+-          break;
+-        }
+-      }
+     }
+ 
+     // 'const', 'pure' and 'noalias' attributed functions are also nounwind.
+--- b/test/CodeGen/alloc-fns-alignment.c
++++ /dev/null
+@@ -1,35 +0,0 @@
+-// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm < %s | FileCheck %s --check-prefix=ALIGN16
+-// RUN: %clang_cc1 -triple x86_64-windows-msvc      -emit-llvm < %s | FileCheck %s --check-prefix=ALIGN16
+-// RUN: %clang_cc1 -triple i386-apple-darwin        -emit-llvm < %s | FileCheck %s --check-prefix=ALIGN16
+-// RUN: %clang_cc1 -triple i386-unknown-linux-gnu   -emit-llvm < %s | FileCheck %s --check-prefix=ALIGN8
+-// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fno-builtin-malloc  -emit-llvm < %s  | FileCheck %s --check-prefix=NOBUILTIN-MALLOC
+-// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fno-builtin-calloc  -emit-llvm < %s  | FileCheck %s --check-prefix=NOBUILTIN-CALLOC
+-// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fno-builtin-realloc -emit-llvm < %s  | FileCheck %s --check-prefix=NOBUILTIN-REALLOC
+-
+-typedef __SIZE_TYPE__ size_t;
+-
+-void *malloc(size_t);
+-void *calloc(size_t, size_t);
+-void *realloc(void *, size_t);
+-
+-void *malloc_test(size_t n) {
+-  return malloc(n);
+-}
+-
+-void *calloc_test(size_t n) {
+-  return calloc(1, n);
+-}
+-
+-void *raalloc_test(void *p, size_t n) {
+-  return realloc(p, n);
+-}
+-
+-// ALIGN16: align 16 i8* @malloc
+-// ALIGN16: align 16 i8* @calloc
+-// ALIGN16: align 16 i8* @realloc
+-// ALIGN8: align 8 i8* @malloc
+-// ALIGN8: align 8 i8* @calloc
+-// ALIGN8: align 8 i8* @realloc
+-// NOBUILTIN-MALLOC: declare i8* @malloc
+-// NOBUILTIN-CALLOC: declare i8* @calloc
+-// NOBUILTIN-REALLOC: declare i8* @realloc