If you have something with high discover-ability like a Firefox exploit(high because a couple have already been found, patched, and people are presumably actively looking), why would govt need to hide this evidence? Seems to me that it doesn't matter how they identified and took control of a given hidden service, what should be relevant to the case is the bit of JS sent to the perpetrator's browser that pings the government server revealing the user's real IP.
What am I missing? Is it that that exploit has somehow gone unnoticed and is so valuable that it can't be released? Is it that using exploits to gain evidence is not admissible? Seems weird to me.