In FOR, we saw a large vessal that sniped ships from a great distance. This craft used a continuos cutting-like beam. Speculation from the board so far is that we have not seen this weapon before or that the Covenant may have invented it. The truth is we have seen this weapon before and so has the Covenant. This beam is a modification of a Sentinal's weapon. The description of it in FOR and the beam that the Sentinals on Halo use are the same with two key differences, power and color, which I feel are related.
You'll notice that plasma pistols issue a green bolt while plasma rifles issue a blue bolt. While I know of no "in-game" evidence to suggest that one is more powerful than the other, the power difference between a pistol and a rifle is implied. The sniper beam used by the large craft is also blue which may indicate that there is a shift in the color spectrum as an energy weapon's power level increases. The Sentinal's beam is red-orange and is fairly weak compared to the plasma rifle and pistol.