Hi Dragon, the problem in this case is your cutting object has gotten a bit messy, it is made up of a lot of different pieces that are kind of overlapping over top of each other.

This is the problem piece:



If you select it an run Edit/Separate on it, you can then drag the faces that make it up, and you can see that they're kind of bigger pieces that overlap over top of each other:



That kind of overlapped surfaces inside one single object is not good, it will confuse the booleans.

To fix it, go in there and select the 5 bottom edges of the current shape:



And then do Edit/Copy. Now delete the whole object and then do Edit/Paste - this is the basic way to extract an edge from a solid as an independent curve. That should leave you with these curves:



Now run Construct / Extrude to punch those curves up into a new solid. This new solid will have a streamlined simple structure where each side is just one single surface - it will now work as a cutting object for your boolean.



Hopefully that should let some smoke out of your firepit! :)

- Michael