Best Firefox Addons Extensions:

1) NoScript : With this you are completely safe from malware and spyware scripts. (Though it may become a annoyance):

2)PicLens: Lets you view all images on selective sites as 3D WALL or slideshow.

3)FlashGot: Extension For directing url and browser referrer to your selected Download Manager.

4)M R Tech Local Install Lot of customization options , good ones like local theme install.

5)Ad-Block Plus: In Association with Filterset -G is a Powerful ad blocker.

6)All-In-One Sidebar: Collection of all major options in a neat tidy sidebar.

7)Video Download Helper : Used for downloading Videos from various streaming websites.

8 )Wordpess.com Sidebar: Get quick access to your wordpress.com account from your browser sidebar.

9)Megauplad SX 3 : Used to bypass megaupload country slot .

10)GreaseMonkey: Very controversial extension used to implement bits of javascript to change the looks of some webpages. (Orkut , Rapidshare , Megaupload).