I use this with Firebug all the time - it's a fantastic tool for exploring the jQuery API against the DOM of any web page. A lot of the time I'll use this technique to prototype a new feature "live" for one of my own sites.

I much prefer this over the GreaseMonkey version

You can get jQuerify button into Firebug by using FireQuery extension (http://firequery.binaryage.com).

Yes, a bookmarklet is more general solution, which works for every browser. In FireQuery it is just a convenience bonus feature.