Looks like it's spreading backwards into that wall of yours. AFAIK, fire can only spread 2-3 blocks, but it doesn't care if there's anything between those blocks, so it's spreading through your brick to the log wall.
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I made a particle accelerator out of raw and cooked ham, but all I ended up finding was the Piggs Bacon
The only things wood are good for are tools and flooring; if you're constructing a non-treehouse structure out of anything but stone, you deserve having it burn to cinders. Not only is stone far more prevalent in the world, it's also infinitely more resistant to random lapses in forethought!
What's a fireplace good for, anyway? The only thing a furnace can't cook is a dwelling!
edit: A fireplace might add ambiance to a dwelling, but still pales in comparison to domesticated lava-pools, IMO.
If you build the fireplace first light it, then wait a bit for the fire code to go static, you can start building around the fireplace again. A word of caution though, don't modify any blocks around the fireplace later or it can cause the game to reset the fire state and check for flammables again.
In your picture, remove all the wood blocks over the fireplace and the back wall behind it. Then light the fire.
One issue I think you will encounter is that with such a large fireplace the fire will consume some of those logs you intend to keep burning. Will take several trys to get them all burning forever.
Pretend to be weak, that he may grow arrogant. If he is taking his ease, give him no rest. If his forces are united, separate them. Attack him where he is unprepared, appear where you are not expected. Art of war