Project https://github.com/Eclipse-Community/r3dfox as FF clone works on Win 7! Not tested on XP nor Vista, yet.
Latest offline installer of FF is at https://download-installer.cdn.mozilla.net/pub/firefox/releases/121.0/win64/en-GB/
Unpacked FF 121 works on Win 8.x by downgrade MajorOperatingSystemVersion/MajorSubsystemVersion in firefox.exe Optional Header (10 > 4)
Unpacked FF 121 works on Win 7 by borrowing libs from r3dfox (ex vyvfox) project of the same version. Libs are:
lgpllibs.dll
mozglue.dll
xul.dll
For further version, bithack might be considered for those 3 libs and 32bit version, also.