Compiled on PI4B4G with 5.10.77-piCore-v8 on SD card F2FS filesystem (10G clean partition, 6.5G available after extraction)

References:
Firefox options
https://wiki.mozilla.org/Linux/Compiler_Options
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
Enable LTO with both C/C++ and Rust (Cross-language LTO)
https://bugzilla.mozilla.org/show_bug.cgi?id=1546438
 --> ERROR: Cross-language LTO is not supported with GCC
What is Cross-language LTO
https://blog.llvm.org/2019/09/closing-gap-cross-language-lto-between.html
Increase the Ulimit for linker (not tested)
https://hubicka.blogspot.com/2018/12/firefox-64-built-with-gcc-and-clang.html
https://developers.redhat.com/blog/2020/03/18/cross-language-link-time-optimization-using-red-hat-developer-tools#example__firefox

Compile Flags:
CC='clang -march=armv8-a+crc -mtune=cortex-a72 -L/usr/local/lib'
CXX='clang++ -march=armv8-a+crc -mtune=cortex-a72 -L/usr/local/lib'
LDFLAGS='-Wl,-O1'
export CC CXX LDFLAGS

Dependencies: (pixz is optional, but tar is required)
# before loading dependencies, block icu just in case
$ touch /usr/local/tce.installed/icu
$ tce-load -i ...

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

Commands:

# LLVM setup
# check which LTO plugin we are using
# not using the GCC one, liblto_plugin.so
# 1 file only, remove any other plugins that might be picked up
ls /usr/local/lib/bfd-plugins/
LLVMgold.so

# replace strip with llvm
sudo ln -sfv /usr/local/bin/llvm-strip /usr/local/bin/strip

# make sure /usr/local/bin/llvm-config is a binary
sudo cp /tmp/tcloop/llvm-dev/usr/local/bin/llvm-config /usr/local/bin/llvm-config

# source code
# F2FS partition root directory is writable by average user
time tar -I pixz -xf /mnt/sda1/compile/firefox/firefox-91.7.0esr.source.tar.xz -C /mnt/mmcblk0p3/
real    3m 50.44s
user    0m 59.99s
sys     0m 43.21s

mv /mnt/mmcblk0p3/firefox-91.7.0 /mnt/mmcblk0p3/firefox-lto
cd /mnt/mmcblk0p3/firefox-lto

time tar -I pixz -xf /mnt/mmcblk0p2/compile/firefox-aarch64/firefox-91.7.0esr.source.tar.xz -C /mnt/sda1/

real    15m 9.82s
user    1m 3.72s
sys     0m 53.28s

mv /mnt/sda1/firefox-91.7.0 /mnt/sda1/firefox-cross
cd /mnt/sda1/firefox-cross

# This patch is required for LTO build
patch -Np1 -i /tmp/firefox-91.7.0esr-disable_rust_test-1.patch

# export variables
# llvm binaries
AR='llvm-ar'
NM='llvm-nm'
RANLIB='llvm-ranlib'
export AR NM RANLIB

# PATH and compile flags
# you need to tell the mach build system to search library in /usr/local/lib
PATH='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
CC='clang -march=armv8-a+crc -mtune=cortex-a72 -L/usr/local/lib'
CXX='clang++ -march=armv8-a+crc -mtune=cortex-a72 -L/usr/local/lib'
LDFLAGS='-Wl,-O1'
export CC CXX LDFLAGS PATH

# Miscellaneous
# Tell Rust to optimize for size
RUSTC_OPT_LEVEL='s'
MOZBUILD_STATE_PATH="${PWD}/mozbuild"
MACH_USE_SYSTEM_PYTHON='1'
export RUSTC_OPT_LEVEL MOZBUILD_STATE_PATH MACH_USE_SYSTEM_PYTHON

# mozconfig cross language LTO
cp /tmp/mozconfig.cross mozconfig

# mach build
# just in case, a 'make clean' equivalent
time ./mach clobber
real    0m 9.17s
user    0m 6.46s
sys     0m 1.45s

# configure
time ./mach configure
 1:35.08 Total wall time: 45.57s; CPU time: 36.86s; Efficiency: 81%; Untracked: 1.27s
Configure complete!
Be sure to run |mach build| to pick up any changes
real    1m 37.69s
user    1m 8.45s
sys     0m 21.97s

# make
# if you need verbose output --> time ./mach -v --log-file /tmp/firefox.log build
time ./mach build
real    10h 46m 03s
user    29h 44m 06s
sys     32m 4.55s

DESTDIR=/tmp/packing-firefox time ./mach -v install
real    0m 28.68s
user    0m 13.87s
sys     0m 6.11s

# for future reference (might be affected by libpng)
du -bhd0 /tmp/packing-firefox
185.8M  /tmp/packing-firefox

# success build with libpng
du -bhd0 /tmp/packing-firefox/
185.7M  /tmp/packing-firefox/

# add a soft link so ondemand can load this directly
cd /tmp/packing-firefox/usr/local/bin
ln -s /usr/local/lib/firefox/firefox firefox-perf