Having just spent nearly an hour debugging this with Neona, I’ve found a couple of things:

Macs, at least, don’t like it if you set minflezz to 10 million or so. The default 100k works just fine. Suck up the potential loss.

As Master_Troll discovered a year ago, logging out and back in again can help immensely. I wish I’d gone back through the whole thread again.

And finally, it might (or might not) help if your only extension is Greasemonkey and your only script is this one. I haven’t iteratively re-added all my other scripts yet to see if things break. I went to the extent of blowing away my Firefox install, ~/Library/Application Support/Firefox and ~/Library/Caches/Firefox, then installing clean downloads of FF 3.5.6 / latest Greasemonkey / latest Blackjack script. All worked til I edited that damn minflezz. Log in to see images!