Tweaking a Greasemonkey script is easy. It’s just a single file, so you download the file locally, edit it, and install it the same way you’d install any other script.
I did this because I needed a quick fix for the super-helpful XMLHttpRequest Debugging script. Sometimes the console has a little trouble with positioning – using it with Google Maps caused it to sit behind the upper portion of the page due to layering. So I made two quick fixes – increased the z-index in the embedded stylesheet so it would appear in front and also changed the default coordinates (I’d adjusted them with “about:config”, but somehow that wasn’t picked up.)
All in all, I was able to tweak the script, not knowing anything about the GM API, and have it running in my browser in about 10 minutes. Had it been a standard Firefox extension, I would have been out of luck. I’d presumably have to download the original source, set up a dev/testing environment, and be able to package it all up including meta-info. Furthermore, I’d have to restart Firefox to test it, unlike Greasemonkey which works straight away. I’ve never tried all that with extensions, but that’s my perception from a little looking around.
I’m nowhere near as bullish as some about Greasemonkey, at least in the medium-term, as some people, because I think the whole Firefox extension mechanism is way too complex for most end-users, let alone the idea that you have to install Greasemonkey scripts on top of one of those extensions. But in any event, once you have the Greasemonkey extenstion installed, it’s a cinch to remould a script you come across.