Re: "Of course, we know that thoughts are generated by the firing of neurons in the brain"
Well, given that neuronal firing is a synonym for the rapid movement of ions across a neuronal membrane, there would be a shift in mass involved.
However, a swift kick to the head to dislodge some stuck thoughts doesn't usually achieve the desired results.
But we have to experimentally validate to see if that is actually is the case, and in the process we might indeed find a value for the mass of a thought, by applying standard kicks to the head (calibrated in NorrisLinguini) and averaging the number of unstuck thoughts resulting from that.