After many years of using Firefox I have finally started using the grease monkey add-on.  This is the description directly from the addon page.

Allows you to customize the way a webpage displays using small bits of JavaScript.
Hundreds of scripts, for a wide variety of popular sites, are already available at http://userscripts.org.
You can write your own scripts, too. Check out http://wiki.greasespot.net/ to get started. 

 As it states it allows you to write your own java script and apply it to existing web pages.  JavaScript although simple can be a powerful tool.  If you have your own server space you can even use it as a bridge between server side scripts and client side scripts with the use of ajax.  But you can ignore all of this because now the support is large enough that most people have already made scripts for you to use.  So weather your a regular user or a super duper user you can get a lot of mileage out of greasemonkey.  I would install the add-n  called grease fire.  This script will alert you when scripts are available for the current web page your on.  

Even though I've just started my journey into greasemonkey i have quickly found 2 scripts that are fun and useful.  The first script is for YouTube which automatically makes all videos high quality and also does not allow to automatically play.  

The second script is for Google, it makes all the Google logos into a doodle that i selected.  Google's clever, mysterious, and completely random logo changes, known as "doodles," usually last for just one day. With this Greasemonkey script, you can make any official Google doodle the logo you always see when searching.  Find more about the doodle script here.