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According to Canon website :
the file size for Mac OS X is slightly different. Download only from Canon website and it should be fine.
I got 12.3 MB for the .dmg file, but that file size you get isn't the best way for determining file integrity. After calling someone at Canon for support, they confirmed that the file size was what it was supposed to be, so now I've been able to do the firmware update.
If anyone wants this for future reference, here's the md5sum info for the .FIR file: