To try and bring some clarity here, I'd like to talk a little about what GreaseMonkey is and isn't.
GreaseMonkey isn't a separate program or a 'hack', it's an extension that one can install onto FireFox. It's the same kind of concept as AdBlock or Ghostery. Adblock and Ghostery are pretty simple add-ons - when a page is being loaded into your browser, these extensions look for a particular code (one that displays an ad or flags a beacon) and removes that code before your browser finishes loading the page.
To link to a graphic from AdBlock Plus' website:
What this means is, simply, if I had AdBlock installed, the effects of AdBlock *only* occur on my computer. I'm not changing anything server-side, nor am I changing anything for other users.
To break it down more simply - think of this like television. The television station broadcasts a signal. You - like everyone else - pick up the signal on your TV. Now, a commercial comes on - so you hit the mute button. You're changing the way the television broadcast signal is displayed on your television - but doesn't, in any way, affect how anyone else views or receives the signal.
GreaseMonkey is the *same* concept - but the thing that differentiates it from AdBlock or Ghostery is that individual users can create or add in their own scripts, telling GreaseMonkey what to look for and what to do with it once it is found. AdBlock and Ghostery are pretty defined in what they look for and, for the most part, are controlled remotely by the creators of those extensions. Whereas AdBlock might only look for and remove "X" and Ghostery might only look for and remove "Y", with GreaseMonkey, you can make it so that it looks for and removes "X", "Y", and "Z", but also changes "A" to display twice the normal size, "B" to display in green instead of blue and "C" to be replaced with alternate text.
But, no matter what a user does with GreaseMonkey, those changes will ONLY affect that user (and only on the browser where they install the script - for example, my phone currently doesn't have anything on it, so it displays as it would for every other user). I know how doing something to a computer that another person doesn't understand equates to "hacking" - I've seen it a million times over my years, both in my private and professional life (Side Note: I once got accused of "hacking" into a work website when all I did was click a link that sent a request for me to have access to the site and the assistant to the individual in charge approved it without telling the individual in charge. When the individual in charge found out I had access to the website, he assumed that I "hacked" it because I had access. All I did was click a link that said "Can I come in?"). It's not "hacking" and it's not nefarious - it is merely hitting the mute button on the TV remote when an ad comes on that you don't want to hear. Everyone else can still hear the ad if they choose - or they can even hit the mute button. Heck, they can turn up the volume to the max if they want. :D