The [i]only[/i] issue I personally have with Firebolt nades (and this is coming from someone who mains a Sunsinger) is that the damage ticks too slow. It leaves you or of combat/recovery for a bit too long compared to other grenades that do similar/more damage.
Only thing they should tweak is increasing the speed at which the damage occurs, enabling someone who took the nade and survived/escaped a better chance at recovering quicker.
Otherwise I feel Sunsingers are no more OP than any other class when played right.
[spoiler]*All of this is my [i]opinion[/i] on potential changes*[/spoiler]