Whatever you do, do not use firefox. Nutaku say they support Chrome and Chrome only. However, my better success with Opera makes me wonder what kind of support they really give Chrome, so i'd recommend Opera. As Kotono said, disable hardware acceleration, it was a cause of lag for me for a long while til i figured it out.

I'd recommend reading the beginner guide at the wiki Beginner Guide - Millennium War Aigis Wiki it has some "do not do this unless you want to be crippled" stuff in it from different experienced players, and i recently overhauled it to make it more easy to read.