Greasemonkey is a Firefox extension that allows you to customize the way webpages look and function. 

Hundreds of scripts are already available for free. And if you're the tinkerer sort, you can also write your own. 

Scripts

• Twitter Live Updater: Updates live the twitter home timeline without browser refresh
• digg a twitter : a simple greasemonkey script that adds "digg this" icon to twitters
• Twitter with Meebo: A Twit Box on the Meebo Instant Messenger using GreaseMonkey. Allows quick twittering from Meebo - By Parashuram.
• twitter mod: twitter UI and experiences improved.
• Twitter Preview: This script displays your friend's last twitter when you mouseover their image in the sidebar.
• twitter_lite: a twitter client
• Twitter Followers Manager: your twitter's followers manager.(Twitter の Followers ページから add/remove を管理)
• HatenaB with Twitter: hatena bookmark with twitter.(はてなブックマーク時に Twitter へクロスポスト)
• post2sbm: This script provides a post bookmark UI for del.icio.us, hatena-bookmark, and twitter.
• twitter friend name helper: Show list of friend names when you type "@" or "D".
• twitter: a twitter client
• Twitter - Add follow & leave link:
• Browse with Twitter: Update your twitter.com status with a message "Browsing: [document.title]" whenever you load a web page.
• twitter profile popup: When the cursor is put together on the friend icon, the profile is displayed.
• Twitter Balatarin: The script adds a "post to balatarin" button under each twitter message.
• Twitter Display Name Mod: Switches the username with the display name on Twitter.com.
• Twitter Highlight: Highlights tweets that are to you in your timeline (i.e. tweets that start with @your_username).
• Twitter Hashtags: Auto-links hashtags in tweets.
• Twitter Search: adds a search interface (using the newly acquired Summize) to the standard twitter pages (see a screenshot)
• Twitter Friends' Bio at a glance: See bio text and web link for all friends and followers on the Twitter follow or friends list
• Twitter Display Popular Trends: Display popular Twitter Trends on your Twitter home page.
• Twitter Auto Replace URL with bit.ly URL: Automatically replace any Tweet URL with bit.ly URL as you type or paste in a URL.
• Retweet This: Add a "RT" link to every tweet on Twitter.com for one-click retweeting.
• @troynt's Twitter Script: Very comprehensive. Adds Nested Replies, Custom Search Tabs, Autocomplete, Pagination, RT button, Media Embed, URL Expansion, Hash Tag Search Links, Social Links and User Groups. Actively developed. 
• Twitter Trends Explained: Adds a tooltip to each 'Trending Topic' to let you know what the trend is all about (using the 'What The Trend?' service).
• twitter_ua: The Ukrainian twitter.