Installer deletes files
From MozillaZine Knowledge Base
NOTE: Please use a build later than February 6, 2004, or you will encounter the following bug.
If you use the installer file "FirefoxSetup.exe" (0.8 branch or trunk) dating from before February 6, 2004, be warned that you can accidentally erase data from your hard drive.
If you select the default installation, there is no problem. Firefox installs in "C:\Program Files\Mozilla Firefox\".
If you select custom installation, and mark the checkbox "Clean Install", Firefox setup will delete all the files in the install directory. Thus, if you somehow select to install into "C:\Program Files\" -- instead of "C:\Program Files\Firefox\" -- all your files in "C:\Program Files\" will be deleted. Windows's folder selection user interface makes this easy to happen.
When using Custom Install, the option to create a new folder exists only in Windows XP and 2000. In all other OSes, you will have to create a folder before launching installer if you want to install into a folder other than the default.
It is strongly recommended you use builds newer than February 6, 2004 to avoid this bug!