diff --git a/build/autoconf/clang-plugin.m4 b/build/autoconf/clang-plugin.m4
index dbc35e82b429..e64602c10c15 100644
--- a/build/autoconf/clang-plugin.m4
+++ b/build/autoconf/clang-plugin.m4
@@ -36,11 +36,7 @@ if test -n "$ENABLE_CLANG_PLUGIN"; then
     dnl For some reason the llvm-config downloaded from clang.llvm.org for clang3_8
     dnl produces a -isysroot flag for a sysroot which might not ship when passed
     dnl --cxxflags. We use sed to remove this argument so that builds work on OSX
-    dnl
-    dnl For a similar reason, we remove any -gcc-toolchain arguments, since the
-    dnl directories specified by such arguments might not exist on the current
-    dnl machine.
-    LLVM_CXXFLAGS=`$LLVMCONFIG --cxxflags | sed -e 's/-isysroot [[^ ]]*//' -e 's/-gcc-toolchain [[^ ]]*//'`
+    LLVM_CXXFLAGS=`$LLVMCONFIG --cxxflags | sed -e 's/-isysroot [[^ ]]*//'`
 
     LLVM_LDFLAGS=`$LLVMCONFIG --ldflags | tr '\n' ' '`
 
diff --git a/build/build-clang/build-clang.py b/build/build-clang/build-clang.py
index 01bd75aa6fb2..99a5ed129425 100755
--- a/build/build-clang/build-clang.py
+++ b/build/build-clang/build-clang.py
@@ -120,23 +120,8 @@ def delete(path):
             pass
 
 
-def install_libgcc(gcc_dir, clang_dir, is_final_stage):
-    gcc_bin_dir = os.path.join(gcc_dir, 'bin')
-
-    # Copy over gcc toolchain bits that clang looks for, to ensure that
-    # clang is using a consistent version of ld, since the system ld may
-    # be incompatible with the output clang produces.  But copy it to a
-    # target-specific directory so a cross-compiler to Mac doesn't pick
-    # up the (Linux-specific) ld with disastrous results.
-    #
-    # Only install this for the bootstrap process; we expect any consumers of
-    # the newly-built toolchain to provide an appropriate ld themselves.
-    if not is_final_stage:
-        x64_bin_dir = os.path.join(clang_dir, 'x86_64-unknown-linux-gnu', 'bin')
-        mkdir_p(x64_bin_dir)
-        shutil.copy2(os.path.join(gcc_bin_dir, 'ld'), x64_bin_dir)
-
-    out = subprocess.check_output([os.path.join(gcc_bin_dir, "gcc"),
+def install_libgcc(gcc_dir, clang_dir):
+    out = subprocess.check_output([os.path.join(gcc_dir, "bin", "gcc"),
                                    '-print-libgcc-file-name'])
 
     libgcc_dir = os.path.dirname(out.rstrip())
@@ -324,7 +309,7 @@ def build_one_stage(cc, cxx, asm, ld, ar, ranlib, libtool,
     build_package(build_dir, cmake_args)
 
     if is_linux():
-        install_libgcc(gcc_dir, inst_dir, is_final_stage)
+        install_libgcc(gcc_dir, inst_dir)
     # For some reasons the import library clang.lib of clang.exe is not
     # installed, so we copy it by ourselves.
     if is_windows():
@@ -691,13 +676,9 @@ if __name__ == "__main__":
     elif is_linux():
         extra_cflags = []
         extra_cxxflags = []
-        # When building stage2 and stage3, we want the newly-built clang to pick
-        # up whatever headers were installed from the gcc we used to build stage1,
-        # always, rather than the system headers.  Providing -gcc-toolchain
-        # encourages clang to do that.
-        extra_cflags2 = ["-fPIC", '-gcc-toolchain', stage1_inst_dir]
+        extra_cflags2 = ["-fPIC"]
         # Silence clang's warnings about arguments not being used in compilation.
-        extra_cxxflags2 = ["-fPIC", '-Qunused-arguments', '-gcc-toolchain', stage1_inst_dir]
+        extra_cxxflags2 = ["-fPIC", '-Qunused-arguments']
         extra_asmflags = []
         # Avoid libLLVM internal function calls going through the PLT.
         extra_ldflags = ['-Wl,-Bsymbolic-functions']
diff --git a/build/unix/build-gcc/build-gcc.sh b/build/unix/build-gcc/build-gcc.sh
index d8497c4f5bbe..1190e7fe09d1 100755
--- a/build/unix/build-gcc/build-gcc.sh
+++ b/build/unix/build-gcc/build-gcc.sh
@@ -39,10 +39,7 @@ build_binutils() {
     #
     # --enable-targets builds extra target support in ld.
     # Enabling aarch64 support brings in arm support, so we don't need to specify that too.
-    #
-    # It is important to have the binutils --target and the gcc --target match,
-    # so binutils will install binaries in a place that gcc will look for them.
-    binutils_configure_flags="--enable-targets=aarch64-unknown-linux-gnu --build=x86_64-unknown-linux-gnu --target=x86_64-unknown-linux-gnu --disable-gold --enable-plugins --disable-nls --with-sysroot=/"
+    binutils_configure_flags="--enable-targets=aarch64-unknown-linux-gnu --disable-gold --enable-plugins --disable-nls --with-sysroot=/"
   fi
 
   mkdir $root_dir/binutils-objdir
@@ -55,13 +52,9 @@ build_binutils() {
 }
 
 build_gcc() {
-  # Be explicit about --build and --target so header and library install
-  # directories are consistent.
-  local target="${1:-x86_64-unknown-linux-gnu}"
-
   mkdir $root_dir/gcc-objdir
   pushd $root_dir/gcc-objdir
-  ../gcc-$gcc_version/configure --prefix=${prefix-/tools/gcc} --build=x86_64-unknown-linux-gnu --target="${target}" --enable-languages=c,c++  --disable-nls --disable-gnu-unique-object --enable-__cxa_atexit --with-arch-32=pentiumpro --with-sysroot=/
+  ../gcc-$gcc_version/configure --prefix=${prefix-/tools/gcc} --enable-languages=c,c++  --disable-nls --disable-gnu-unique-object --enable-__cxa_atexit --with-arch-32=pentiumpro --with-sysroot=/
   make $make_flags
   make $make_flags install DESTDIR=$root_dir
 
diff --git a/taskcluster/ci/fetch/toolchains.yml b/taskcluster/ci/fetch/toolchains.yml
index 4f65662f1360..70969bc2b659 100644
--- a/taskcluster/ci/fetch/toolchains.yml
+++ b/taskcluster/ci/fetch/toolchains.yml
@@ -20,6 +20,17 @@ binutils-2.27:
       sig-url: "{url}.sig"
       key-path: build/unix/build-gcc/EAF1C276A747E9ED86210CBAC3126D3B4AE55E93.key
 
+binutils-2.28.1:
+  description: binutils 2.28.1 source code
+  fetch:
+    type: static-url
+    url: ftp://ftp.gnu.org/gnu/binutils/binutils-2.28.1.tar.xz
+    sha256: 16328a906e55a3c633854beec8e9e255a639b366436470b4f6245eb0d2fde942
+    size: 19440112
+    gpg-signature:
+      sig-url: "{url}.sig"
+      key-path: build/unix/build-gcc/EAF1C276A747E9ED86210CBAC3126D3B4AE55E93.key
+
 binutils-2.31.1:
   description: binutils 2.31.1 source code
   fetch:
@@ -39,6 +50,17 @@ cloog-0.18.1:
     sha256: 02500a4edd14875f94fe84cbeda4290425cb0c1c2474c6f75d75a303d64b4196
     size: 3857324
 
+gcc-4.9.4:
+  description: GCC 4.9.4 source code
+  fetch:
+    type: static-url
+    url: ftp://ftp.gnu.org/gnu/gcc/gcc-4.9.4/gcc-4.9.4.tar.bz2
+    sha256: 6c11d292cd01b294f9f84c9a59c230d80e9e4a47e5c6355f046bb36d4f358092
+    size: 90097606
+    gpg-signature:
+      sig-url: "{url}.sig"
+      key-path: build/unix/build-gcc/13975A70E63C361C73AE69EF6EEB81F8981C74C7.key
+
 gcc-6.4.0:
   description: GCC 6.4.0 source code
   fetch:
diff --git a/taskcluster/ci/toolchain/clang-tidy.yml b/taskcluster/ci/toolchain/clang-tidy.yml
index 4ba0dc08a655..f4b96191f209 100644
--- a/taskcluster/ci/toolchain/clang-tidy.yml
+++ b/taskcluster/ci/toolchain/clang-tidy.yml
@@ -34,7 +34,7 @@ linux64-clang-tidy:
             - 'build/build-clang/clang-tidy-linux64.json'
             - 'taskcluster/scripts/misc/tooltool-download.sh'
     toolchains:
-        - linux64-gcc-6
+        - linux64-gcc-4.9
 
 macosx64-clang-tidy:
     index:
@@ -55,7 +55,7 @@ macosx64-clang-tidy:
     toolchains:
         - linux64-cctools-port
         - linux64-clang-7
-        - linux64-gcc-6
+        - linux64-gcc-4.9
         - linux64-node
 
 win32-clang-tidy:
diff --git a/taskcluster/ci/toolchain/clang.yml b/taskcluster/ci/toolchain/clang.yml
index f917991d0d40..a68385e362de 100644
--- a/taskcluster/ci/toolchain/clang.yml
+++ b/taskcluster/ci/toolchain/clang.yml
@@ -22,7 +22,7 @@ linux64-clang-4.0:
             - 'taskcluster/scripts/misc/tooltool-download.sh'
         toolchain-artifact: public/build/clang.tar.xz
     toolchains:
-        - linux64-gcc-6
+        - linux64-gcc-4.9
 
 linux64-clang-7:
     description: "Clang 7 toolchain build"
@@ -38,7 +38,7 @@ linux64-clang-7:
         toolchain-alias: linux64-clang
         toolchain-artifact: public/build/clang.tar.xz
     toolchains:
-        - linux64-gcc-6
+        - linux64-gcc-4.9
 
 linux64-clang-trunk-mingw-x86:
     description: "MinGW-Clang Trunk x86 toolchain build"
@@ -54,7 +54,7 @@ linux64-clang-trunk-mingw-x86:
             - 'taskcluster/scripts/misc/tooltool-download.sh'
         toolchain-artifact: public/build/clangmingw.tar.xz
     toolchains:
-        - linux64-gcc-6
+        - linux64-gcc-4.9
 
 linux64-clang-trunk-mingw-x64:
     description: "MinGW-Clang Trunk x64 toolchain build"
@@ -71,7 +71,7 @@ linux64-clang-trunk-mingw-x64:
             - 'taskcluster/scripts/misc/tooltool-download.sh'
         toolchain-artifact: public/build/clangmingw.tar.xz
     toolchains:
-        - linux64-gcc-6
+        - linux64-gcc-4.9
 
 linux64-clang-7-android-cross:
     description: "Clang 7 toolchain build"
@@ -86,7 +86,7 @@ linux64-clang-7-android-cross:
         toolchain-alias: linux64-clang-android-cross
         toolchain-artifact: public/build/clang.tar.xz
     toolchains:
-        - linux64-gcc-6
+        - linux64-gcc-4.9
         - linux64-android-ndk-linux-repack
 
 linux64-clang-7-macosx-cross:
@@ -109,7 +109,7 @@ linux64-clang-7-macosx-cross:
     toolchains:
         - linux64-cctools-port
         - linux64-clang-7
-        - linux64-gcc-6
+        - linux64-gcc-4.9
 
 macosx64-clang:
     description: "Clang toolchain build"
@@ -133,7 +133,7 @@ macosx64-clang:
     toolchains:
         - linux64-cctools-port
         - linux64-clang-7
-        - linux64-gcc-6
+        - linux64-gcc-4.9
         - linux64-node
 
 win64-clang-cl:
diff --git a/taskcluster/ci/toolchain/gcc.yml b/taskcluster/ci/toolchain/gcc.yml
index 5f255d153885..0b14126b147a 100644
--- a/taskcluster/ci/toolchain/gcc.yml
+++ b/taskcluster/ci/toolchain/gcc.yml
@@ -11,6 +11,22 @@ job-defaults:
             - 'build/unix/build-gcc/build-gcc.sh'
         toolchain-artifact: public/build/gcc.tar.xz
 
+linux64-gcc-4.9:
+    description: "GCC 4.9 toolchain build"
+    treeherder:
+        symbol: TL(gcc4.9)
+    run:
+        script: build-gcc-4.9-linux.sh
+    fetches:
+        fetch:
+            - binutils-2.25.1
+            - cloog-0.18.1
+            - gcc-4.9.4
+            - gmp-5.1.3
+            - isl-0.12.2
+            - mpc-0.8.2
+            - mpfr-3.1.5
+
 linux64-gcc-6:
     description: "GCC 6 toolchain build"
     treeherder:
@@ -20,7 +36,7 @@ linux64-gcc-6:
         toolchain-alias: linux64-gcc
     fetches:
         fetch:
-            - binutils-2.31.1
+            - binutils-2.28.1
             - gcc-6.4.0
             - gmp-5.1.3
             - isl-0.15
@@ -35,7 +51,7 @@ linux64-gcc-7:
         script: build-gcc-7-linux.sh
     fetches:
         fetch:
-            - binutils-2.31.1
+            - binutils-2.28.1
             - gcc-7.3.0
             - gmp-6.1.0
             - isl-0.16.1
@@ -55,7 +71,7 @@ linux64-gcc-sixgill:
         - linux64-gcc-6
     fetches:
         fetch:
-            - binutils-2.31.1
+            - binutils-2.28.1
             - isl-0.15
             - gcc-6.4.0
             - gmp-5.1.3
diff --git a/taskcluster/ci/toolchain/misc.yml b/taskcluster/ci/toolchain/misc.yml
index a1208540c83e..efeceecbd3f4 100644
--- a/taskcluster/ci/toolchain/misc.yml
+++ b/taskcluster/ci/toolchain/misc.yml
@@ -32,7 +32,7 @@ linux64-llvm-dsymutil:
         script: build-llvm-dsymutil.sh
         toolchain-artifact: public/build/llvm-dsymutil.tar.xz
     toolchains:
-        - linux64-gcc-6
+        - linux64-gcc-4.9
 
 linux64-binutils:
     description: "Binutils toolchain build"
@@ -85,7 +85,7 @@ linux64-tup:
         - trunk
         - try
     toolchains:
-        - linux64-gcc-6
+        - linux64-gcc-4.9
 
 linux64-custom-v8:
     description: "Custom v8 build"
diff --git a/taskcluster/scripts/misc/build-clang-trunk-mingw.sh b/taskcluster/scripts/misc/build-clang-trunk-mingw.sh
index 5a8331a2ab4c..79271f056c32 100755
--- a/taskcluster/scripts/misc/build-clang-trunk-mingw.sh
+++ b/taskcluster/scripts/misc/build-clang-trunk-mingw.sh
@@ -321,6 +321,7 @@ popd
 mkdir -p $UPLOAD_DIR
 
 pushd $(dirname $INSTALL_DIR)
+rm -f clang/lib/libstdc++*
 tar caf clangmingw.tar.xz clang
 mv clangmingw.tar.xz $UPLOAD_DIR
 popd
diff --git a/taskcluster/scripts/misc/build-gcc-6-linux.sh b/taskcluster/scripts/misc/build-gcc-6-linux.sh
index 6a9908266636..1f198074384e 100755
--- a/taskcluster/scripts/misc/build-gcc-6-linux.sh
+++ b/taskcluster/scripts/misc/build-gcc-6-linux.sh
@@ -14,7 +14,7 @@ data_dir=$HOME_DIR/src/build/unix/build-gcc
 
 gcc_version=6.4.0
 gcc_ext=xz
-binutils_version=2.31.1
+binutils_version=2.28.1
 binutils_ext=xz
 
 pushd $root_dir/gcc-$gcc_version
diff --git a/taskcluster/scripts/misc/build-gcc-7-linux.sh b/taskcluster/scripts/misc/build-gcc-7-linux.sh
index 8b9674215fd5..5862c1033d14 100755
--- a/taskcluster/scripts/misc/build-gcc-7-linux.sh
+++ b/taskcluster/scripts/misc/build-gcc-7-linux.sh
@@ -14,7 +14,7 @@ data_dir=$HOME_DIR/src/build/unix/build-gcc
 
 gcc_version=7.3.0
 gcc_ext=xz
-binutils_version=2.31.1
+binutils_version=2.28.1
 binutils_ext=xz
 
 pushd $root_dir/gcc-$gcc_version
diff --git a/taskcluster/scripts/misc/build-gcc-sixgill-plugin-linux.sh b/taskcluster/scripts/misc/build-gcc-sixgill-plugin-linux.sh
index 7aba4219b822..3e42065415be 100755
--- a/taskcluster/scripts/misc/build-gcc-sixgill-plugin-linux.sh
+++ b/taskcluster/scripts/misc/build-gcc-sixgill-plugin-linux.sh
@@ -28,7 +28,7 @@ sixgill_repo=https://hg.mozilla.org/users/sfink_mozilla.com/sixgill
 . $data_dir/build-gcc.sh
 
 pushd $root_dir/gcc-$gcc_version
-ln -sf ../binutils-2.31.1 binutils
+ln -sf ../binutils-2.28.1 binutils
 ln -sf ../gmp-5.1.3 gmp
 ln -sf ../isl-0.15 isl
 ln -sf ../mpc-0.8.2 mpc