Is a copy of JIRA going to be kept around somewhere, ideally
web-readable (static HTML?) or just as database backup?
I don't understand if you're only speaking of open or also closed
tickets. Would you want to first do a triaging on JIRA to see what's
still current and migrate the rest, or migrate everything?
I can't help with scripts and I think all the designated tickets must be
"dumped" into bugzilla automatically for later triaging; I can help with
the triaging at whatever stage, let me know.