Nothing is more Pagan than “The Fire”. But unless it is locally sourced sustainably, firewood can contribute to deforestation. Manufactured fire logs, however, are a great alternative- and not just because they're so hassle-free, but also because they're typically made from recycled wood chips and sawdust, recycled cardboard, or coffee grounds deterred from landfills. The binding agent is typically a wax base, and many of them use paraffin. So, look for ones that use a vegetable-based wax or a recycled wax like with Duraflame, Enviro-Log, or PineMountain's Java Log.
And though natural firewood does contain captured carbon within, from when it was a tree, the release of its carbon into our atmosphere is almost nothing in comparison to other domestic contributors.