diff --git a/projects/firefox/Dockerfile b/projects/firefox/Dockerfile
index 1c10f9d01..39725c61e 100644
--- a/projects/firefox/Dockerfile
+++ b/projects/firefox/Dockerfile
@@ -15,14 +15,16 @@
 ################################################################################
 
 FROM gcr.io/oss-fuzz-base/base-builder
+RUN add-apt-repository -y ppa:ubuntu-toolchain-r/test
 RUN apt-get update && apt-get install -y --no-install-recommends \
+  libstdc++6 \
   python \
   gawk \
   software-properties-common
-RUN add-apt-repository -y ppa:ubuntu-toolchain-r/test
-RUN apt-get update && apt-get upgrade -y && apt-get install -y --no-install-recommends \
-  libstdc++6
 RUN git clone --depth 1 https://github.com/mozilla/gecko-dev mozilla-central
 RUN git clone --depth 1 https://github.com/mozillasecurity/fuzzdata
 WORKDIR mozilla-central
 COPY build.sh target.c *.options mozconfig.* $SRC/
+# Install dependencies.
+ENV SHELL /bin/bash
+RUN ./mach bootstrap --no-interactive --application-choice browser
diff --git a/projects/firefox/build.sh b/projects/firefox/build.sh
index 62718d553..431cfb8a6 100755
--- a/projects/firefox/build.sh
+++ b/projects/firefox/build.sh
@@ -42,9 +42,7 @@ FUZZ_TARGETS=(
 export MOZ_OBJDIR=$WORK/obj-fuzz
 export MOZCONFIG=$SRC/mozconfig.$SANITIZER
 
-# Install dependencies.
 export SHELL=/bin/bash
-./mach bootstrap --no-interactive --application-choice browser
 
 # Skip patches for now
 rm tools/fuzzing/libfuzzer/patches/*.patch
diff --git a/projects/firefox/mozconfig.coverage b/projects/firefox/mozconfig.coverage
index 8178d9026..a99a37728 100644
--- a/projects/firefox/mozconfig.coverage
+++ b/projects/firefox/mozconfig.coverage
@@ -1,3 +1,4 @@
+mk_add_options AUTOCLOBBER=1
 ac_add_options --disable-debug
 ac_add_options --disable-elf-hack
 ac_add_options --disable-jemalloc