The page for Lexar drivers is
here. The only FireWire CF reader they list is on their
discontinued products page. It lists compatibility with OS 8.6 - OS 9, but no OSX drivers. If it really is their FireWire product and not a USB unit, you may be out of luck on that one.
The page for Memorex drivers is
here. Same story oddly, OS9 drivers, but no OSX driver. OSX usually doesn't need a driver for USB mass storage devices. Open Apple System Profiler with the USB CF reader already plugged in. Check the USB section to see what your G5 can see. Also check with Disk Utility, see if it sees the drive, but thinks it's unformatted.
It may matter what the compact flash card is formatted as. Cameras and small flash drives typically format as FAT16. Should work as FAT32. If it were formatted as NTFS, I'm not sure how the G5 will like it. Last I heard, NTFS mounts as read-only with hard drives under OSX. I have not tested under Tiger to see if the driver has been upgraded.