Compiled on {PI4B4G, RPI3B} cluster with 5.10.77-piCore-v8 via crossover cable on external SSD using F2FS filesystem
To setup such cluster, you'll need a crossover cable, just link them together.

References:
Crossover connection
https://www.linuxquestions.org/questions/linux-networking-3/howto-crossover-connection-between-two-linux-machines-110270/#post569094
Firefox options
https://wiki.mozilla.org/Linux/Compiler_Options
Distcc setup (Important: 'mk_add_options MOZ_MAKE_FLAGS="CFLAGS=..."' doesn't work)
https://wiki.cdot.senecacollege.ca/wiki/Distcc_mozilla_tutorial
This FLAG helps in size, defined in mozxonfig (-finline-limit=64)
https://pcxfirefox.wordpress.com/2016/04/16/firefox-on-android-some-compiler-and-linker-switch-onoff/
Tell Rust to optimize for size
https://firefox-source-docs.mozilla.org/setup/configuring_build_options.html
https://doc.rust-lang.org/rustc/codegen-options/index.html#opt-level

In my case, setup crossover cluster:
pi3 - $ sudo ifconfig eth0 10.0.0.1
pi4 - $ sudo ifconfig eth0 10.0.0.2

Swap space:
pi3: 4G
pi4: 10G

Distcc setup: pi3 is volunteer, pi4 is worker

On pi3 (Server machine)

# kill the running daemon
sudo pkill distccd

# make sure to remove it, not sure why, but it may not log if you don't delete it
sudo rm /var/log/distccd.log
sudo touch /var/log/distccd.log
sudo chmod 666 /var/log/distccd.log

# export compile flags before you start the demon
# They should be passed by distcc, but just in case
CFLAGS='-march=armv8-a+crc -mtune=cortex-a72 -Os'
CXXFLAGS='-march=armv8-a+crc -mtune=cortex-a72 -Os'
LDFLAGS='-Wl,-O1'
export CFLAGS CXXFLAGS LDFLAGS

# start the daemon
sudo distccd --daemon --user tc --allow 10.0.0.2 --jobs 4 --verbose --log-file=/var/log/distccd.log
  # You should see something like this
  distccd[8221] (dcc_discard_root) successfully set no_new_privs
  distccd[8221] (dcc_discard_root) discarded root privileges, changed to uid=1001 gid=50

On pi4 (Client machine)

# kill the running daemon
sudo pkill distccd

# edit ~/.distcc/hosts
# 4 jobs each will not do
# pi3 is the bottle neck, the ram clot up easily
# by the time pi3 finally complete, pi4 already move on to the next target
localhost/4
10.0.0.1/3

# verify
distcc --show-hosts

Compile Flags: ('-Os' is passed in mozconfig, '-pipe' is added automatically)
CC='gcc -march=armv8-a+crc -mtune=cortex-a72'
CXX='g++ -march=armv8-a+crc -mtune=cortex-a72'
LDFLAGS='-Wl,-O1'
export CC CXX LDFLAGS

Dependencies: (better to load on both machines)
# before loading dependencies, block icu just in case
$ touch /usr/local/tce.installed/icu
$ tce-load -i ...

distcc
autoconf
cbindgen
dbus-glib-dev
libevent-dev
llvm-dev
gtk3-dev
libnotify-dev
icu70-dev
node-dev
nss-dev
libasound-dev
alsa
alsa-utils
startup-notification-dev
zip
unzip
libwebp-dev
clang
libXt-dev
curl-dev
tar
pixz

Commands:

# Both machines
sudo /usr/local/bin/ln -s /tmp/tcloop/nss-dev/usr/local/include/nss/* -t /usr/local/include

# from now on, do everything on pi4

time tar -I pixz -xf firefox-91.7.0esr.source.tar.xz
real    1m 20.65s
user    0m 57.63s
sys     0m 40.35s

cd firefox-91.7.0

# You won't need firefox-91.5.1esr-buildfix-1.patch
# It has already been patched

# firefox-91.6.0esr-disable_rust_test-1.patch
# We can pass this test, so it's not really that necessary
# It'll probably save some time, but I'm not applying

cp ../mozconfig.distcc mozconfig

# export variables
# Tell Rust to optimize for size
export RUSTC_OPT_LEVEL='s'
export MOZBUILD_STATE_PATH="${PWD}/mozbuild"
export MACH_USE_SYSTEM_PYTHON='1'
export PATH='/usr/lib/distcc:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
# make sure you have export the 'Compile Flags' above

# just in case, a 'make clean' equivalent
time ./mach clobber
real    0m 8.13s
user    0m 6.23s
sys     0m 1.44s

# configure
time ./mach configure
 1:35.68 Total wall time: 47.19s; CPU time: 37.85s; Efficiency: 80%; Untracked: 1.06s
Configure complete!
Be sure to run |mach build| to pick up any changes
real    1m 38.25s
user    1m 10.03s
sys     0m 21.60s

# make
# if you need verbose output --> time ./mach -v build
time ./mach build
437:11.11 If you are building Firefox often, SCCache can save you a lot of time. You can learn more here: https://firefox-source-docs.mozilla.org/setup/configuring_build_options.html#sccache
To take your build for a test drive, run: |mach run|
For more information on what to do now, see https://firefox-source-docs.mozilla.org/setup/contributing_code.html
real    7h 17m 13s
user    25h 11m 24s
sys     42m 20.22s

sudo DESTDIR=/tmp/packing-firefox MACH_USE_SYSTEM_PYTHON=1 time ./mach -v install
real    1m 8.71s
user    0m 35.14s
sys     0m 13.00s

unsquashfs old-firefox.tcz usr/local/share

sudo cp -r squashfs-root/usr/local/share /tmp/packing-firefox/usr/local/

cd /tmp

mksquashfs packing-firefox/ /mnt/mmcblk0p2/tce/optional/upgrade/firefox.tcz -quiet