Just made it playing around LOL, though it works =o
Short way: Download the .exe and run it =)
Long way for the paranoid who think the .exe will be a virus or something: download the rmrk.xml file and put it in the \searchplugins folder in your firefox directory. This is all the .exe does ~
Whichever way you install it, firefox needs a restart before it'll appear in the search list up the top right