This firefox extension wraps the gpMin Chrome extension and has been tested in Firefox 3.6, 4, and 5.
Installing this software could bring about the Apocalypse, harm your unborn child, cause rickets and generally cause all manner of bad things. Do so at your own risk.
But in all seriousness, I wrote gpMin in a day and the wrapper in a morning – it may or may not work for you.
Note: The only reason this extension isn’t on addons.mozilla.org is because it uses some deprecated events that they don’t like. I’m not looking to change the code as gpMin is a stop gap until Google changes the UI themselves (which I’m sure they will).