This is an excellent addition and one which I used successfully before the geocaching.com changes.
Unfortunately the current version no longer works on a Mac (OS 10.4.9, Firefox 2.0.0.4, GreaseMonkey 0.6.9.200070507.0) and fails in a rather spectacular manner. After installing, all other GreaseMonkey scripts disappear. I have not managed to find the source of the problem but am still looking into it because I would really like to use it again. I will, however, share how to recover once you have tried to install it after my long and painful investigation. So after having tried to install the script and found that although it appears to install, at lest the first time you try, it does not work, the easiest way to recover is to delete the folder ~/Library/Application Support/Firefox You will of course loose all you Firefox add-ons, bookmarks etc but I have found no other way to recover without having to completely reinstall Firefox.
Of course if anyone has managed to use it successfully on a Mac or can suggest anything I should try, please let me know.