1) Download Status Bar
This Addon is one of the best because it offers ease of use when downloading files. With custom functionality it is a lot better than FireFox's built in down loader.2) Add Block Plus
This extension is one of the best for Firefox, gives the ability to block any URL. A must have if you find adverts annoying.3) FoxMarks
This program is some what unique as it automatically (and securely) uploads you bookmarks, which can be retrieved and used on other computers.4) Firebug
Don't you find it annoying when you click a button and nothing happens, are a page loads incorrectly. This extension can help. It shows HTML, javascript and other errors when surfing the web. Useful to have when things go wrong.5) Adsense Preview
Do you have Google Adsense Disorder (GAD) where you have to check your Adsense earning every 10 seconds. This extension is for you. It has the ability to automatically check how much you have earned. A very useful tool.6) Screen Grab!
This add on gives the ability to take a image of a website, much better than a Print Screen job, you would normally have to do.7) Grease Monkey
One of the best extensions and should be included with every release of Firefox. Gives the user scripting ability to change the why a web page works to suit you needs.8) Tab Effect
More of a cool widget thing that improving functionality, gives cool tab transitions.9) Colour Full Tabs
Again another extension that offers the more colour and life into Firefox. I love it.
10) Mouse Gestures
A useful extension that enables you to do mouse gestures (Duh) to do particular commands, back forward to name a few.You can download all of the Add Ons Here
