I extracted firefox and digged into "config.ini"
i found the "-ma" switch for auto install with GUI mode which shows the installation progress,
and the "-ms" switch for totaly silent mode.

These are also working with the unextracted install file.

So i edited a config.txt like
;!@Install@!UTF-8!
RunProgram="FIREFOX.EXE -ms"
;!@InstallEnd@!

Made a 7z file without compression(because it's allready good compressed)
from "Firefox Setup 1.0.2.exe" and named it "FireFox.7z"

finally i build a sfx install file:

copy /b 7zs.sfx + config.txt + FireFox.7z FireFox.EXE

And it worked fine & silent

Why not just call the exe with -ms directly instead of repacking it like that?

+1
unless adding plugins, extensions, themes and/or profile, repacking firefox setup is now useless since they fixed the error that prevented the installer to run "properly" with -ms command line switch