OK, I figured out how to do this automatically.
So, since the beginning of time, there have been 9567 bugs in the Browser, MailNews, Firefox and Thunderbird products which started as UNCONFIRMED, got confirmed before they got resolved, and ended up FIXED. This is a table of how long each of them took to get confirmed:
|Elapsed Time||# Confirmed||% Confirmed||# open UNCO older than this|
So the larger data set bears out the results from the smaller one. After 3 months, we’ve squeezed 96.7% of the juice out of the UNCO pile. (And, these figures don’t take into account the case when someone finds a long-dead bug which doesn’t occur any more and marks it FIXED instead of WORKSFORME.) Yet, we have 8200 UNCO bugs open which are older than that.
 This is a technical limitation. There were 11912 overall; i.e. 2345 went straight from UNCO to FIXED. I don’t think leaving them out of the table skews the data, but I’ll listen to arguments that say it does.