As required by law:
Awsome job, great idea!
I have found I can get the compressed portable_firefox_1.5_en-us.zip down from 6,643,139 bytes to 5,258,684 bytes using Tugzip, http://www.tugzip.com/ ,and 7z compression.
This is a significant savings especially for download bandwidth.
Original (download from sourceforge) - 6,643,139 bytes
7z Ultra Compression - 5,258,684
7z & a Self-Extractor - 5,343,676 (79% current file size) 1.29MB saved
I would recommend considering use of 7z and self-extractor to lower your bandwidth overhead.
Disclaimer: I am in now way associated with tugzip or the makers of 7z. I am a recent convert from the zip/tar world. My plans for world domination will continue through my many nefarious compression schemes... Bwa haha!