Download it from the Mozilla Site:
http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/1.0.5/contrib/firefox-os2-1.0.5.zip
NOTE: This release requires the C runtime DLLs from Innotek. The latest version is 0.5.1, which includes an installer. Available here:
http://download.innotek.de/gccos2/runtime/libc-0.5.1.exe
NOTE: This release requires IPLUGINW.DLL to use the Java and Flash plugins. Available here:
http://www.innotek.de/products/javaos2/mozilla/
Source: OS2.jp