CURE FOR SEALING PROBLEM
Several people have commented that their fireSleeve won't stay closed. If yours isn't defective, it's probably because the captured air acts as a spring. The seal is good, so the air doesn't escape - but then it pushes the cap off.
I push the cap on, and then while holding it closed, I twist the rubber body back and forth a few times to distort it. This breaks the seal while the cap is fully on and releases the captured air. After this, the cap remains closed and sealed.
Without the air spring inside, it holds well, too.