Furnaces Unusable

I first downloaded Minecraft about a week ago. Furnaces worked fine. Then starting yesterday, I could not produce anything with a furnace in any of my worlds. I can put in the fuel and the item to be cooked, but then nothing happens. It doesn't matter which item I put in first. I mainly tried this with different combinations of wood and coal. I deleted Minecraft.app and the files in the ~/Library/Application Support/Minecraft folder, and then I re-downloaded and installed the game. Same issue. I have only ever played Single Player Survival Mode, and I've never installed any mods.

The only thing that I did that might have had an effect is that I downloaded a world from the Internet and put it in the ~Library/Application Support/Minecraft/Saves folder. I couldn't get that world to work-it might have been made for a previous version of Minecraft-so I deleted the file. The furnaces stopped working some time after that.

Any help you can provide would be much appreciated!! Thank you.