That's what I thought you might say. (It's been a long time, but I think restoration of pre-KP states is SOP in OS X.)
The only thing I can think of is for you to figure out where Firefox stores its history and clear that file before you reopen it.
Edit:
where does FF store history on hard disk? (/Users/artie/Library/Application Support/Firefox/Profiles/t5wb6yrg.default/places.sqlite [...your specifics, of course.])
From Mozilla's description of the file it's got something to do with bookmarks as well as history, so you may want to save it to your desktop rather than trash it. (I
do see a file labeled "bookmarks.html", so I dunno.)
