Search and download GreaseMonkey from https//addons.mozilla.org/  and once at GreaseMonkey page, press the 'Add to Firefox' button. When the download is complete, press 'Re-start Firefox' button and note that GreaseMonkey is added in 'Tools' menu. Here, one can select 'New User Script' to write scripts from scratch or 'Manage User Scripts' to see which scripts have been added and the websites they customise, etc. Let's start with a simple script before we go on to see some readymade stuff.

Select 'New User Script' and fill in the fields in the window that appears. Entering name 'test' or anything you like in the first three fields namely 'Name', 'Namespace' and 'Description' and leave the 'Include' and 'Exclude' website names to their defaults — press 'OK'. Note that the website currently opened in the existing browser tab automatically appears in 'Includes' list which means this script will work when this site will be loaded.

Now enter alert ('Hello world!') in the text editor after the default comments. Now refresh the web page in the existing tab to see the alert popping up. To learn more about user script programing, take a look at online tutorials such as those available at wiki.greasespot.net in the form of user script reference and JavaScript tutorials as GreaseMonkey is based on JavaScript.

Let us see some useful ready-to-download and use scripts available over Userscripts.org — with its home page having direct tag links to Gmail, Google, YouTube, etc. Take a look at Gmail tag and install script 'Gmail Small Attachment Icons'. This can be done by either going through the list of Gmail scripts or searching for it. Pressing the 'install' button at this script's page leads to GreaseMonkey installation window appearing in browser. Press 'Install' button in this window to complete the installation. This script adds a tiny icon in 'Gmail Inbox' message rows where an attachment is present to identify the type of file such as document or image, etc. Similarly, there are scripts to remove ads from Gmail as well.

Then there are new skins available such as 'professional' to add mouse-over effect and 'colourful' to customise the look and feel of the Gmail interface. Take a look at 'YouTube Enhancer' script and visit Youtube.com/edu — the educational videos site where you can find subject wise lectures from universities around the world and also guitar lessons! What makes this particular 'Enhancer' script useful for students is that it allows users to watch the video in very low quality — useful over slow speed internet connections.

Another useful addition is the 'Download' button at the top right of the video which is not available by default but is useful for offline viewing and sharing.

Once at the website, note the presence of monkey icon at the bottom right in the status bar. Right clicking this link tells which scripts are active right now so at YouTube website, the 'YouTube Enhancer' will appear as checked items. From this icon, entire GreaseMonkey add-on can be disabled which means none of the installed scripts will work. All in all, it's a great tool for enhancing user experience and learning to program.

Link to download
http//wiki.greasespot.net/
http//diveintogreasemonkey.org/