diff --git a/build/moz.configure/finalize-flags.configure b/build/moz.configure/finalize-flags.configure
index c1a0054d072f..73c8760cf9b8 100644
--- a/build/moz.configure/finalize-flags.configure
+++ b/build/moz.configure/finalize-flags.configure
@@ -41,12 +41,23 @@ add_old_configure_assignment("LDFLAGS", os_ldflags)
 set_config("OS_LDFLAGS", os_ldflags)
 
 
-@depends(linker_optimize_flags, moz_optimize_flags, lto, c_compiler)
-def moz_optimize_ldflags(linker_optimize_flags, moz_optimize_flags, lto, c_compiler):
+set_config("MOZ_OPTIMIZE_FLAGS", moz_optimize_flags, when=~js_build)
+
+
+@depends(
+    linker_optimize_flags,
+    "MOZ_OPTIMIZE_FLAGS",
+    lto,
+    c_compiler,
+)
+@imports(_from="mozbuild.shellutil", _import="split")
+def moz_optimize_ldflags(linker_optimize_flags, env_optimize_flags, lto, c_compiler):
     flags = []
     if linker_optimize_flags:
         flags += linker_optimize_flags.ldflags
-    flags += moz_optimize_flags
+
+    if env_optimize_flags:
+        flags += split(env_optimize_flags[0])
 
     # When using llvm-based LTO, non numeric optimization levels are
     # not supported by the linker, so force the linker to use -O2 (
@@ -58,3 +69,20 @@ def moz_optimize_ldflags(linker_optimize_flags, moz_optimize_flags, lto, c_compi
 
 
 set_config("MOZ_OPTIMIZE_LDFLAGS", moz_optimize_ldflags)
+
+
+@depends(
+    try_compile(
+        includes=["stdio.h"],
+        body='puts("demat");',
+        flags=moz_optimize_flags,
+        language="C",
+        check_msg="for valid C compiler optimization flags",
+    ),
+    moz_optimize_flags,
+    when=moz_optimize,
+)
+@imports(_from="mozbuild.shellutil", _import="quote")
+def check_optimization_flags(check_result, moz_optimize_flags):
+    if not check_result:
+        die(f"Invalid C compiler optimization flags: {quote(*moz_optimize_flags)}")
diff --git a/build/moz.configure/toolchain.configure b/build/moz.configure/toolchain.configure
index cda2258981d9..bee1ed4916cb 100644
--- a/build/moz.configure/toolchain.configure
+++ b/build/moz.configure/toolchain.configure
@@ -98,36 +98,42 @@ def forced_pgo_optimization_level(target):
         return "-O3"
 
 
-@depends("--enable-optimize")
-def moz_optimize(option):
-    if len(option):
-        if "-O0" not in option[0]:
-            return "2"
-    elif option:
-        return "1"
-
-
-@depends("--enable-optimize", forced_pgo_optimization_level)
+@depends("--enable-optimize", "MOZ_OPTIMIZE_FLAGS")
 @imports(_from="mozbuild.shellutil", _import="split")
-def moz_optimize_flags(option, forced_pgo_optimization_level):
-    if len(option):
-        return split(option[0])
-    elif option and forced_pgo_optimization_level:
-        return [forced_pgo_optimization_level]
+def configured_moz_optimize_flags(enable_optimize, env_flags):
+    if len(enable_optimize):
+        return split(enable_optimize[0])
+    if len(env_flags):
+        return split(env_flags[0])
+
+
+@depends("--enable-optimize", "MOZ_OPTIMIZE_FLAGS")
+def moz_optimize(enable_optimize, env_flags):
+    return "1" if enable_optimize or env_flags else None
 
 
 set_config("MOZ_OPTIMIZE", moz_optimize)
-add_old_configure_assignment("MOZ_OPTIMIZE", moz_optimize)
-add_old_configure_assignment("MOZ_CONFIGURE_OPTIMIZE_FLAGS", moz_optimize_flags)
 
 
-@depends_if("MOZ_OPTIMIZE_FLAGS")
+@depends(
+    target, moz_optimize, configured_moz_optimize_flags, forced_pgo_optimization_level
+)
 @imports(_from="mozbuild.shellutil", _import="split")
-def moz_optimize_flags(env_optimize_flags):
-    return split(env_optimize_flags[0])
+def moz_optimize_flags(
+    target, moz_optimize, configured_moz_optimize_flags, forced_pgo_optimization_level
+):
+    if configured_moz_optimize_flags:
+        return configured_moz_optimize_flags
 
+    if moz_optimize and forced_pgo_optimization_level:
+        return [forced_pgo_optimization_level]
 
-add_old_configure_assignment("MOZ_OPTIMIZE_FLAGS", moz_optimize_flags)
+    if target.kernel == "Darwin":
+        return ["-O3"]
+    elif target.kernel in ("Linux", "WINNT"):
+        return ["-O2"]
+    else:
+        return ["-O"]
 
 
 # Android NDK
@@ -3878,20 +3884,3 @@ dtrace = check_header(
 
 set_config("HAVE_DTRACE", True, when=dtrace)
 set_define("INCLUDE_MOZILLA_DTRACE", True, when=dtrace)
-
-
-@depends(
-    try_compile(
-        includes=["stdio.h"],
-        body='puts("demat");',
-        flags=moz_optimize_flags,
-        language="C",
-        check_msg="for valid C compiler optimization flags",
-    ),
-    moz_optimize_flags,
-    when=moz_optimize,
-)
-@imports(_from="mozbuild.shellutil", _import="quote")
-def check_optimization_flags(check_result, moz_optimize_flags):
-    if not check_result:
-        die(f"Invalid C compiler optimization flags: {quote(*moz_optimize_flags)}")
diff --git a/js/moz.configure b/js/moz.configure
index 8fb51095876f..08df84d2d165 100644
--- a/js/moz.configure
+++ b/js/moz.configure
@@ -1477,6 +1477,34 @@ with only_when(compile_environment):
     )
 
 
+# Optimization flags
+# ==============================================================
+@depends(
+    target,
+    c_compiler,
+    configured_moz_optimize_flags,
+    when="--enable-compile-environment",
+)
+@imports(_from="mozbuild.shellutil", _import="split")
+def moz_js_optimize_flags(target, compiler, configured_moz_optimize_flags):
+    if configured_moz_optimize_flags:
+        return configured_moz_optimize_flags
+
+    if target.kernel in ("Darwin", "Linux"):
+        flags = ["-O3"]
+        if target.os == "Android" and compiler.type == "gcc":
+            flags += ["-fno-reorder-functions"]
+        return flags
+    elif target.kernel == "WINNT":
+        return ["-O2"]
+    else:
+        return ["-O"]
+
+
+set_config("MOZ_JS_OPTIMIZE_FLAGS", moz_js_optimize_flags, when=~js_build)
+set_config("MOZ_OPTIMIZE_FLAGS", moz_js_optimize_flags, when=js_build)
+
+
 # link executables against mozglue
 # ==============================================================
 @depends(
diff --git a/js/src/old-configure.in b/js/src/old-configure.in
index 9baff651572d..9ea8d8652abf 100644
--- a/js/src/old-configure.in
+++ b/js/src/old-configure.in
@@ -44,53 +44,6 @@ fi
 
 fi # COMPILE_ENVIRONMENT
 
-dnl ========================================================
-dnl System overrides of the defaults for target
-dnl ========================================================
-
-case "$target" in
-*-darwin*)
-    MOZ_OPTIMIZE_FLAGS="-O3"
-    ;;
-
-*-android*|*-linuxandroid*)
-    MOZ_OPTIMIZE_FLAGS="-O3"
-    if test "$CC_TYPE" == gcc ; then
-       MOZ_OPTIMIZE_FLAGS="-fno-reorder-functions $MOZ_OPTIMIZE_FLAGS"
-    fi
-    ;;
-
-*-*linux*)
-    MOZ_OPTIMIZE_FLAGS="-O3"
-    ;;
-
-*-mingw*)
-    MOZ_OPTIMIZE_FLAGS="-O2"
-    ;;
-
-esac
-
-if test -z "$MOZ_OPTIMIZE_FLAGS"; then
-	MOZ_OPTIMIZE_FLAGS="-O"
-fi
-
-
-dnl Mozilla specific options
-dnl ========================================================
-dnl The macros used for command line options
-dnl are defined in build/autoconf/altoptions.m4.
-
-dnl ========================================================
-dnl = Enable code optimization. ON by default.
-dnl ========================================================
-
-# Use value from moz.configure if one is defined. Else use our computed
-# value.
-if test -n "${MOZ_CONFIGURE_OPTIMIZE_FLAGS}"; then
-    MOZ_OPTIMIZE_FLAGS=${MOZ_CONFIGURE_OPTIMIZE_FLAGS}
-fi
-
-AC_SUBST_LIST(MOZ_OPTIMIZE_FLAGS)
 
 dnl ========================================================
 dnl =
diff --git a/old-configure.in b/old-configure.in
index 1af75c496b15..34fff2d451e3 100644
--- a/old-configure.in
+++ b/old-configure.in
@@ -35,39 +35,6 @@ fi
 
 fi # COMPILE_ENVIRONMENT
 
-dnl ========================================================
-dnl System overrides of the defaults for target
-dnl ========================================================
-
-case "$target" in
-*-darwin*)
-    MOZ_OPTIMIZE_FLAGS="-O3"
-    ;;
-
-*-android*|*-linuxandroid*)
-    MOZ_OPTIMIZE_FLAGS="-O2"
-    ;;
-
-*-*linux*)
-    if test "$CC_TYPE" != clang-cl ; then
-        MOZ_OPTIMIZE_FLAGS="-O2"
-    fi
-    ;;
-*-mingw*)
-    MOZ_OPTIMIZE_FLAGS="-O2"
-    ;;
-esac
-
-if test -z "$MOZ_OPTIMIZE_FLAGS"; then
-    MOZ_OPTIMIZE_FLAGS="-O"
-fi
-
-
-dnl Mozilla specific options
-dnl ========================================================
-dnl The macros used for command line options
-dnl are defined in build/autoconf/altoptions.m4.
-
 dnl ========================================================
 dnl =
 dnl = Application
@@ -124,18 +91,6 @@ fi
 
 AC_SUBST(MOZ_BRANDING_DIRECTORY)
 
-dnl ========================================================
-dnl = Enable code optimization. ON by default.
-dnl ========================================================
-
-# Use value from moz.configure if one is defined. Else use our computed
-# value.
-if test -n "${MOZ_CONFIGURE_OPTIMIZE_FLAGS}"; then
-    MOZ_OPTIMIZE_FLAGS=${MOZ_CONFIGURE_OPTIMIZE_FLAGS}
-fi
-
-AC_SUBST_LIST(MOZ_OPTIMIZE_FLAGS)
-
 
 dnl ========================================================
 dnl = Mac bundle name
diff --git a/python/mozbuild/mozbuild/frontend/context.py b/python/mozbuild/mozbuild/frontend/context.py
index 7062c4fb9a07..bef14792cef3 100644
--- a/python/mozbuild/mozbuild/frontend/context.py
+++ b/python/mozbuild/mozbuild/frontend/context.py
@@ -514,6 +514,11 @@ class TargetCompileFlags(BaseCompileFlags):
     def _optimize_flags(self):
         if not self._context.config.substs.get("MOZ_OPTIMIZE"):
             return []
+        # js/src/* have their own optimization flag.
+        if self._context.relsrcdir == "js/src" or self._context.relsrcdir.startswith(
+            "js/src/"
+        ):
+            return self._context.config.substs.get("MOZ_JS_OPTIMIZE_FLAGS")
         return self._context.config.substs.get("MOZ_OPTIMIZE_FLAGS")
 
     def __setitem__(self, key, value):