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2.1.0 - an older, smaller firmware. If you don't need PPPoE you can use this one (i.e. if your ISP is using DHCP, PPPoA, or you have a static IP). It's definitely the most stable firmware. Most "crippled" 5660's ship with a modified version of this firmware which has the router mode disabled.
2.2.0 - this version is obsolete.
2.2.1 - this version is obsolete.
DO NOT flash a 2.2.x firmware on a "small memory" 5260!!
2.3.0 - this is now the Latest Version. It is by far the best firmware Efficient has ever made for the 5660, which honestly isn't saying all that much. Nevertheless, it is the newest version, it fixes bugs in 2.2.x and adds features. It is also more stable, which means you won't need to power-cycle your router as often. Use this version.
At present, Efficient's latest firmware release is 2.3.0(7).
Efficient has not updated their firmware in some time now. The Speedstream 5260/5660 has been superseded by newer models, and both the 5260 and 5660 models have been discontinued.