Instructions for Fedora 20 and 19

First ensure that your compiler toolchain and Gecko build dependencies are installed.

sudo yum install \
  ccache cmake gcc gcc-c++ glibc-devel.i686 \
  libstdc++-devel libstdc++-devel.i686 \
  autoconf213 \
  gtk2-devel.i686 gtk+-devel.i686 gtk+extra-devel.i686 \
  glib-devel.i686 glib2-devel.i686 \
  dbus-devel.i686 dbus-glib-devel.i686 \
  alsa-lib-devel.i686 yasm-devel.i686 \
  gstreamer-devel.i686 gstreamer-plugins-devel.i686 \
  gstreamer-plugins-base-devel.i686 \
  libxml2-devel.i686 zlib-devel.i686 \
  freetype-devel.i686 \
  atk-devel.i686 pango-devel.i686 fontconfig-devel.i686 \
  cairo-devel.i686 gdk-pixbuf2-devel.i686 \
  libX11-devel.i686 libXt-devel.i686 libXext-devel.i686 \
  libXrender-devel.i686 libXau-devel.i686 libxcb-devel.i686 \
  pulseaudio-libs-devel.i686 harfbuzz-devel.i686 \
  mesa-libGL-devel.i686 libXxf86vm-devel.i686 \
  libXfixes-devel.i686 libdrm-devel-2.4.49-2.fc19.i686 \
  mesa-libEGL-devel libXdamage-devel.i686 libXcomposite-devel.i686

Then you need to use a .mozconfig that looks like the following example.

# Flags set for targeting x86.
export PKG_CONFIG_PATH=/usr/lib/pkgconfig

CC="ccache gcc -m32"
CXX="ccache g++ -m32"
ac_add_options --target=i686-pc-linux

# Normal build flags.  These make a debug browser build.
ac_add_options --enable-application=browser
mk_add_options MOZ_MAKE_FLAGS="-s -j6"
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/../ff-dbg

ac_add_options --enable-debug
ac_add_options --disable-optimize

Instructions for Ubuntu 18.10

WARNING: Although Ubuntu and other Debian-derived distributions support multiarch packages these days, there are still cases where packages are broken, or where the 32-bit and 64-bit versions of a package conflict. This makes it difficult to impossible to have a 32-bit development environment co-exist with a 64-bit development environment. So it is recommended to follow these instructions in a separate virtual machine!

These steps were verified to work as of December 2018:

  1. Set up a new virtual machine running Ubuntu 18.10.
  2. Install python with sudo apt install python.
  3. Install mercurial or git and fetch a copy of mozilla-central.
  4. Run ./mach bootstrap to install some dependencies. Note that this will install some amd64 packages that are not needed to build 32-bit Firefox.
  5. Run rustup target install i686-unknown-linux-gnu to install the 32-bit Rust target.
  6. Install 32-bit dependencies with the following command (this may uninstall some unneeded/conflicting packages that were installed by mach bootstrap):
    sudo apt install gcc-multilib g++-multilib libdbus-glib-1-dev:i386 \
      libgtk2.0-dev:i386 libgtk-3-dev:i386 libpango1.0-dev:i386 libxt-dev:i386
  7. Create a .mozconfig file containing at least the following:
    ac_add_options --target=i686-pc-linux
    CFLAGS="$CFLAGS -march=pentium-m -msse -msse2 -mfpmath=sse"
    CXXFLAGS="$CXXFLAGS -march=pentium-m -msse -msse2 -mfpmath=sse"
    export PKG_CONFIG_PATH=/usr/lib/i386-linux-gnu/pkgconfig
  8. Run ./mach build.

Older, generic instructions for Ubuntu

Method 1: True Cross-Compiling

This method is actually cross-compiling: you take a 64-bit toolchain and produce 32-bit binaries. This is ideally how you cross-compile.

  1. sudo apt-get install ia32-libs gcc-multilib g++-multilib lib32*
  2. (I had to open Synaptic afterwards, search for package names beginning with lib32, and install the ones that apt-get missed)
  3. Use a .mozconfig like below.
  4. make -f build
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/objdir-ff-dbg32
mk_add_options MOZ_MAKE_FLAGS="-s -j4"
ac_add_options --enable-application=browser

CC="gcc -m32"
CXX="g++ -m32"
ac_add_options --x-libraries=/usr/lib32
ac_add_options --target=i686-pc-linux

ac_add_options --disable-crashreporter # needed because I couldn't find a 32-bit curl-dev lib
ac_add_options --disable-libnotify     # needed because I couldn't find a 32-bit libinotify-dev
ac_add_options --disable-gnomevfs      # needed because I couldn't find a 32-bit libgnomevfs-dev
ac_add_options --disable-gstreamer     # needed because I couldn't find a 32-bit gstreamer lib

If you are getting an error as follows:

error: Can't find header fontconfig/fcfreetype.h

Add these configuration options to your .mozconfig:

ac_add_options --disable-freetypetest
ac_add_options --disable-pango

Method 2: Create a 32-bit chroot Environment

In this method, we effectively create a wholly-contained 32-bit operating system within a 64-bit operating system using schroot. This isn't technically cross-compiling, but it yields the same result: 32-bit binaries.

This method is arguably more reliable than true cross-compiling because the newly-created environment is completely isolated from the 64-bit operating system and it won't be susceptible to common issues with cross-compiling, such as unavailability of 32-bit libraries/packages when running in 64-bit mode. Additionally, since your 32-bit environment is completely isolated, to clean up from it, you just rm -rf the chroot directory. Contrast this with removing dozens of 32-bit packages from your primary operating system.

The downside to this method is size and complexity. Since you will be effectively creating a whole operating system within your primary operating system, there will be lots of redundant files. You'll probably need at least 1GB for all the new files. Additionally, the steps for initially creating the 32-bit environment are more involved. See the bottom of this page for a script capable of automating the whole process of cross-compilation.

To create a 32-bit chroot Ubuntu environment, follow the DebootstrapChroot instructions. Here is an example config file which works in Ubuntu 13.10:

# /etc/schroot/chroot.d/saucy_i386

description=Ubuntu 13.10 for i386

Once you have changed the root-users and users entries to include your username and verified that $ schroot -c saucy_i386 works, $ exit back to your regular operating system and copy your APT's sources list to the new environment:

$ sudo cp /etc/apt/sources.list /srv/chroot/saucy_i386/etc/apt/sources.list

Note: this assumes a generic sources list. If you have modified this file yourself, you may wish to ensure the contents are accurate when you perform the copy.

The reason we copy the APT sources is because debootstrap does not appear to configure all the sources by default (it doesn't define the "sources" sources, for example).

Once your sources list is copied over, enter your new environment and configure things:

# Update the APT sources and install sudo into the new environment and exit back out
$ schroot -c saucy_i386 -u root
(precise_i386) # apt-get update
(precise_i386) # apt-get install sudo
(precise_i386) # exit

# re-enter the environment as a regular user
$ schroot -c saucy_i386

# Install Firefox build dependencies
$ sudo apt-get build-dep firefox

Now, your new 32-bit operating system should be ready for building Firefox!

One last step is ensuring that configure detects the proper system type. Since you are technically running on a 64-bit kernel, things could still be fooled.
 Run the following program from your mozilla source tree:

$ ./build/autoconf/config.guess

If this prints anything with x86_64, the system type is being incorrectly detected and you must override it. You can fix things by adding the following to your mozconfig:

ac_add_options --host=i686-pc-linux-gnu
ac_add_options --target=i686-pc-linux-gnu

When you run configure ($ mach configure), verify that the host, target, and build system types are what you just defined in your mozconfig:

checking host system type... i686-pc-linux-gnu
checking target system type... i686-pc-linux-gnu
checking build system type... i686-pc-linux-gnu

If you intend to run the 32bits Firefox build in the chroot on the 64bits machine, you need to install a few packages in the host:

sudo apt-get install gcc-multilib g++-multilib libxrender1:i386 libasound-dev:i386 libdbus-glib-1-2:i386 libgtk2.0-0:i386 libxt-dev:i386

Now, follow the build instructions like normal and you should have 32-bit builds!

See also