Firefox Extensions I use & really recommend.

1:-Fasterfox
Why?-Performance and network tweaks for Firefox.
Fasterfox allows you to tweak many network and rendering settings such as simultaneous connections, pipelining, cache, DNS cache, and initial paint delay.
Dynamic speed increases can be obtained with the unique prefetching mechanism, which recycles idle bandwidth by silently loading and caching all of the links on the page you are browsing.
A popup blocker for popups initiated by Flash plug-ins is also included.

2:-Adblock
Why?-Once installed, it's a snap to filter elements at their source-address.
Just right-click: Adblock: done. Filters use either the wildcard character (*)
or full Regular Expression syntax. Hit the status-element and see what has or
hasn't been blocked.

3:-Adblock Filterset.G Updater
Why?-This is a companion extension to Adblock and should be used in conjunction with it. This extension automatically downloads the latest version of Filterset.G every 4-7 days. Filterset.G is an excellent set of filters maintained by G for Adblock that blocks most ads on the internet. In addition, this extension allows you to define your own set of filters that you can add along with Filterset.G during an update.

4:-DownThemAll!
Why?-Why do we have to load a slow external download manager/accelerator, when we can just have DownThemAll inside Firefox?

DownThemAll lets you download all the links or images contained in a webpage and much more: you can refine your downloads by fully customizable criteria to get only what you really want.
DownThemAll is all you can desire from a download manager: it features an advanced accelerator that increases speed up to 400%, it allows you to pause and resume downloads at any time and, last but not least, it's fully integrated into your favorite browser!
No AdWare, no waits. Just the files you want, in a click.

5:-GooglePreview
Inserts preview images (thumbnails) of web sites and Amazon products into the Google and Yahoo search results pages.

6:-Gmail Space
This extension allows you to use your Gmail Space (2 GB) for file storage. It acts as a remote machine. You can transfer files between your hard drive and gmail. This is similar to "Gmail Drive" on windows platform.