diff --git a/build-engine.sh b/build-engine.sh
index 92b9537..7541a81 100755
--- a/build-engine.sh
+++ b/build-engine.sh
@@ -10,6 +10,7 @@ mode="${1:-release}"
 mozconfig="${working_dir}/mozconfig-${mode}"
 objdir="obj-$mode"
 outdir="$mode"
+rebuild="${REBUILD_ENGINE:-0}"
 
 cat << EOF > "$mozconfig"
 ac_add_options --enable-project=js
@@ -62,47 +63,51 @@ case "$mode" in
     ;;
 esac
 
-
-# Ensure the Rust version matches that used by Gecko, and can compile to WASI
-rustup target add wasm32-wasi
-
-fetch_commits=
-if [[ ! -a gecko-dev ]]; then
-
-  # Clone Gecko repository at the required revision
-  mkdir gecko-dev
-
-  git -C gecko-dev init
-  git -C gecko-dev remote add --no-tags -t wasi-embedding \
-    origin "$(cat "$script_dir/gecko-repository")"
-
-  fetch_commits=1
+# For a full build (not a rebuild), we need to clone the repo and do some setup work.
+# `rebuild.sh` invokes this script with REBUILD_ENGINE=1 which sets rebuild=1
+# and skips this setup.
+if [[ $rebuild == 0 ]]; then
+    # Ensure the Rust version matches that used by Gecko, and can compile to WASI
+    rustup target add wasm32-wasi
+
+    fetch_commits=
+    if [[ ! -a gecko-dev ]]; then
+
+      # Clone Gecko repository at the required revision
+      mkdir gecko-dev
+
+      git -C gecko-dev init
+      git -C gecko-dev remote add --no-tags -t wasi-embedding \
+        origin "$(cat "$script_dir/gecko-repository")"
+
+      fetch_commits=1
+    fi
+
+    target_rev="$(cat "$script_dir/gecko-revision")"
+    if [[ -n "$fetch_commits" ]] || \
+      [[ "$(git -C gecko-dev rev-parse HEAD)" != "$target_rev" ]]; then
+      git -C gecko-dev fetch --depth 1 origin "$target_rev"
+      git -C gecko-dev checkout FETCH_HEAD
+    fi
+
+    # Use Gecko's build system bootstrapping to ensure all dependencies are
+    # installed
+    cd gecko-dev
+    ./mach --no-interactive bootstrap --application-choice=js --no-system-changes
+
+    # ... except, that doesn't install the wasi-sysroot, which we need, so we do
+    # that manually.
+    cd ~/.mozbuild
+    python3 \
+      "${working_dir}/gecko-dev/mach" \
+      --no-interactive \
+      artifact \
+      toolchain \
+      --bootstrap \
+      --from-build \
+      sysroot-wasm32-wasi
 fi
 
-target_rev="$(cat "$script_dir/gecko-revision")"
-if [[ -n "$fetch_commits" ]] || \
-  [[ "$(git -C gecko-dev rev-parse HEAD)" != "$target_rev" ]]; then
-  git -C gecko-dev fetch --depth 1 origin "$target_rev"
-  git -C gecko-dev checkout FETCH_HEAD
-fi
-
-# Use Gecko's build system bootstrapping to ensure all dependencies are
-# installed
-cd gecko-dev
-./mach --no-interactive bootstrap --application-choice=js --no-system-changes
-
-# ... except, that doesn't install the wasi-sysroot, which we need, so we do
-# that manually.
-cd ~/.mozbuild
-python3 \
-  "${working_dir}/gecko-dev/mach" \
-  --no-interactive \
-  artifact \
-  toolchain \
-  --bootstrap \
-  --from-build \
-  sysroot-wasm32-wasi
-
 cd "$working_dir"
 
 # Build SpiderMonkey for WASI
diff --git a/rebuild.sh b/rebuild.sh
index 98e94df..e1a7e8c 100755
--- a/rebuild.sh
+++ b/rebuild.sh
@@ -1,86 +1,4 @@
 #!/usr/bin/env bash
 
-set -euo pipefail
-set -x
-
-working_dir="$(pwd)"
-script_dir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
-
-mode="${1:-release}"
-mozconfig="${working_dir}/mozconfig-${mode}"
-objdir="obj-$mode"
-outdir="$mode"
-
-cat << EOF > "$mozconfig"
-ac_add_options --enable-project=js
-ac_add_options --enable-application=js
-ac_add_options --target=wasm32-unknown-wasi
-ac_add_options --without-system-zlib
-ac_add_options --without-intl-api
-ac_add_options --disable-jit
-ac_add_options --disable-shared-js
-ac_add_options --disable-shared-memory
-ac_add_options --disable-tests
-ac_add_options --disable-clang-plugin
-ac_add_options --enable-jitspew
-ac_add_options --enable-optimize=-O3
-ac_add_options --enable-js-streams
-ac_add_options --enable-portable-baseline-interp
-ac_add_options --prefix=${working_dir}/${objdir}/dist
-mk_add_options MOZ_OBJDIR=${working_dir}/${objdir}
-mk_add_options AUTOCLOBBER=1
-EOF
-
-target="$(uname)"
-case "$target" in
-  Linux)
-    echo "ac_add_options --disable-stdcxx-compat" >> "$mozconfig"
-    ;;
-
-  Darwin)
-    echo "ac_add_options --host=aarch64-apple-darwin" >> "$mozconfig"
-    ;;
-
-  *)
-    echo "Unsupported build target: $target"
-    exit 1
-    ;;
-esac
-
-case "$mode" in
-  release)
-    echo "ac_add_options --disable-debug" >> "$mozconfig"
-    ;;
-
-  debug)
-    echo "ac_add_options --enable-debug" >> "$mozconfig"
-    ;;
-
-  *)
-    echo "Unknown build type: $mode"
-    exit 1
-    ;;
-esac
-
-# Build SpiderMonkey for WASI
-MOZCONFIG="${mozconfig}" \
-MOZ_FETCHES_DIR=~/.mozbuild \
-CC=~/.mozbuild/clang/bin/clang \
-CXX=~/.mozbuild/clang/bin/clang++ \
-AR=~/.mozbuild/clang/bin/llvm-ar \
-  python3 "${working_dir}/gecko-dev/mach" \
-  --no-interactive \
-    build
-
-# Copy header, object, and static lib files
-rm -rf "$outdir"
-mkdir -p "$outdir/lib"
-
-cd "$objdir"
-cp -Lr dist/include "../$outdir"
-
-while read -r file; do
-  cp "$file" "../$outdir/lib"
-done < "$script_dir/object-files.list"
-
-cp js/src/build/libjs_static.a "wasm32-wasi/${mode}/libjsrust.a" "../$outdir/lib"
+export REBUILD_ENGINE=1
+exec $(dirname $0)/build-engine.sh "$@"