Firefox Custom builds/forks:

Firefox Custom builds/forks:

before Quantum era:

Windows:
ArcticFox: https://forum.eclipse.cx/viewtopic.php?p=204#p204
Basilisk: https://basilisk-browser.org/
Borealis Navigator: https://binaryoutcast.com/projects/borealis/
Centaury: https://github.com/Feodor2/Centaury/
Cyberfox: https://sourceforge.net/projects/cyberfox/
IceApe: https://o.rthost.win/hbl-uxp/
K-Meleon: http://kmeleonbrowser.org/
Light: https://sourceforge.net/projects/lightfirefox/
Mypal: https://github.com/Feodor2/Mypal/
New Moon: https://o.rthost.win/palemoon/
Palemoon Box: https://sourceforge.net/projects/pale-moon-box/
Palemoon: https://palemoon.org/
pcxFirefox: https://sourceforge.net/projects/pcxfirefox/
ru-board additional custom PM builds: https://forum.ru-board.com/topic.cgi?forum=5&topic=50547&start=0&limit=1&m=5#1
SeaMonkey: https://seamonkey-project.org/
Serpent: https://o.rthost.win/basilisk/
Waterfox Classic: https://waterfox.net/

OS X:
ArcticFox: https://github.com/rmottola/Arctic-Fox
RPM Builds: http://rpm-mozilla.org.uk/builds/firefox/
tenFOURfox: https://floodgap.com/software/tenfourfox/

Quantum era:

Windows:
Comodo IceDragon: https://comodo.com/home/browsers-toolbars/icedragon-browser.php
GNU IceCat (Unofficial builds for Windows x64): https://github.com/muslayev/icecat-win64
Iceweasel (Unofficial builds for Windows x64): https://github.com/muslayev/iceweasel-win64
lawlietfox: https://sourceforge.net/projects/lawlietfox/
libportable Iceweasel: https://sourceforge.net/projects/libportable/files/Iceweasel/
LibreWolf: https://librewolf-community.gitlab.io
tete009: http://www1.plala.or.jp/tete009/en-US/software.html#FIREFOX
TorBrowser: https://dist.torproject.org/torbrowser/
Waterfox: https://waterfox.net/

OS X:
Firefox Quantum for 10.7—10.8: https://forums.macrumors.com/threads/firefox-quantum-ported-to-10-7-10-8.2191887/