My NSLU2 came with the ancient firmware version v2.3R24. The newest available firmware is
V2.3R63, although dead links can be found at the linksys site to V2.3R76.
I have tried to upgrade using the web interface, but it did not work, just the status led started blinking green/amber for hours, until switch-off (in theory, the upgrade should have been finished in 5-10 minutes). After restart the unit came up with the old firmware.