Here is the offending container. It still smelled of mildew after three hard washings (I have reason to believe the friend I lent it to left cookie crumbs sitting in it for a month before washing it).
Frustrated, I remembered something my mother told me about sunlight being a purifying agent and so I let the darned thing sit on my windowsill in the sun for three days. My windowsill also has the heater for my room under it, so the container was subjected to both directly sunlight and constant dry heat. That was a few days ago. Now it smells not of mildew but of sunlight, if sunlight had a smell. Anyway, it smells clean and nice–though I will let it sit for a few more days and then wash out the collected dust before I use it again. Horray for domestic solutions!
“You may have to fight a battle more than once to win it.” Margaret Thatcher (probably not talking about Tupperware)