I think it makes enough sense considering the fact that firebenders have heat control in general. Firebenders are shown that they have the ability to heat up objects, including metal and water (to make tea!)
There's a fine line between firebending and lavabending, but I'd like to extrapolate on that.
For starters, we all know lava is rock that's molten hot. The way a firebender can heat or cool things, to me, means that they could create lava out of rocks if they really wanted to, though I imagine it'd take some training, HOWEVER, they cannot manipulate the object they've heated up, only control its heat.
And, if anything, this would be a nice thing to add to server that would simply prefer to keep their server as nice and natural as possible, because we all know how troublesome it is when a waterbender floods the land or an earthbender makes earth jut out of every thing. The way lava in The Nether is disrupted into troublesome rock formations is something that shouldn't be turned away from in discussion.
