GreaseMonkey one of the most popular firefox addon that enables one to customize a webpage displays using user created javascripts. There are enormous scripts available on userscripts to mod many popular sites like Orkut, IMDB, twitter, Youtube and many more.
Talking about the GM Compiler, it is an online tool which could convert a gm script(with extension .user.js) into a firefox addon(with extension.xpi). After this the gm script extension would change to .xpi , hence would become a firefox addon and would be easy to distribute further.
The Grease Monkey Compiler’s some parameters are pre-filled like a random GUID , firefox min and max compatible versions. All you need to do is to copy and paste the source code of the Greasemonkey script into the User Script text form on the website. 
The tool will create a .xpi file, which could lately be used to install on firefox and work the same as it works like the user script.
On a small review over the same, i came to know that, the script was originally developed by arantius.com, and lately the source of the script was published publicly for webmasters. If you want to make or install the same script on your server, you can get the source code by clicking here.
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Pretty nice tool
TFS!!!!
I have a user script that is used with an online game (yes, its an autobot script shame on me), I have converted it to a firefox add-on so I now dont have to run greasemonkey with firefox (game programmers have a method of detecting greasemonkey without fail and ban you if found using it)
The script in .js form runs several jobs at once when you set them to run, but as an add-on it will only run a max of one job at a time.
I havae had the add-on installed and running for over two weeks without detection, whereas when I just had greasemonkey and the .js I got banned several times on the account
The .js could be set to run multiple jobs to do things, but the add-on, even though you can set it to run multiple will only run one at a time, so presently I am running seven firefox profiles at a time and each profile has a copy of the add-on installed so they dont conflict with each other.
So, my question is “can an add-on be made to work like a .js script and be as responsive to change”