Greasemonkey Add-ons is the right for you. Whats good about it? It is useful for everybody.
This plugin extension allows users to install some scripts that make big changes to HTML webpage, which changes immediately after refreshing the browser.
You can also add new function to web pages, merge data from multiple web pages, fixing rendering bugs, many more purposes.
There are a lot of scripts available and you can find them at http://userscripts.org.
Install - Greasemonkey Plugin for Firefox