Several Useful Firefox Extensions

Drag and Drop Upload
Useful if you frequently upload files to the internet for example photos to Flickr, Gmail or Blogger, this extension allows you to just drag and drop files into the upload fields in web forms, instead of browsing into folders.
Draganddropupload.

Stealther
Switch Firefox to stealth-mode by activating this extension. It temporarily disables:

• Browsing history
• Cookies
• Downloaded Files History
• Disk Cache
• Saved Form Information
• Sending of ReferrerHeader

Extremely easy to install, and activate by clicking Stealther under Tools or press Ctrl+Shift+S. Gina from Lifehacker affectionately calls the Stealther "porn mode", LOL. Please be warned though, with Stealther activated some websites may not work; for example online banking, email accounts and some online accounts that require login. Use only when you don't want to be detected (for whatever reason is up to you ... *cough*Bittorrent*cough*).
Stealther.

Bookmark Duplicate Detector
Isn't it annoying when your bookmarks folder becomes cluttered especially when the clutter results from bookmarks you've entered again and again (a sign that you're 1. getting old and, 2. have dementia :p). Well, this extension detects duplicate URL and will warn you of duplicate bookmarks.
Bookmark Duplicate Detector

RSS Ticker
Display RSS feeds in an attractive and unobtrusive scrolling "ticker" in the right under the status bar (that's a small bar right at the bottom of the Firefox window). Attractive, simple and useful. However, the scrolling news text may distract your reading! (easily remediable by switching off RSS ticker when you need the extra concentration).
RSS Ticker