Not a defender of them but they have specific plantations for growing the fuel just for this. So the idea is that it’s a closed loop. Carbon is captured from the air as the trees grow and then that very same carbon is burnt for energy generation. So no extra carbon is added to the atmosphere when you consider the whole cycle.
Compare with coal and gas where carbon is dug up from under the ground and added to the atmosphere and never removed again.
And if I really really really wanted one in my browser (which I don’t but for the sake of argument) I’ll look for an add-on.
Same reason I don’t want my browser to filter or ban my content but I totally use unlock origin.