The latest firmware for this drive is 2.1B firmware.
There is an unofficial modified 2.1B firmware by Liggy which adds booktype bitsetting for DVD+R media (improving compatibility with older DVD players). You can get it here.
There have been other firmware modifications of earlier firmware that increased supported write speed for some media and perhaps improved write quality of some media, but no such modifications exist for the latest 2.1B firmware.
I would use Liggy’s 3500 bitsetting 2.1B firmware unless you absolutely positively need to have disc quality scanning on your NEC ND-3500AG, in which case you can use Liggy’s 3500 bitsetting 2.R8 firmware available from the same link as above; the 2.R8 firmware has older writing strategies and don’t support as many different DVD media as 2.1B, however.
The Liggy modifications also include faster ripping (but more noisy playback) and RPC1 patching (a.k.a. region-free).
Oh, and welcome to CDFreaks!