Some updates of my favorite Add-ons: (I use many add-ons, sometimes i disable them and try new ones)

Anyway here goes,

Paste Email (original):
A cool add-on which can be used to fill up text in forms with a right-click.
Note:Once installed, go to add-ons and select Paste email and click on options, enter the email addresses or usernames or your physical mailing address in the text boxes.
Now you can use them at any time, for signing in to your accounts or signing up for any website without the hassle of typing.

DownThemAll A download manager that is actually integrated IN firefox, easy to use and incredibly fast.Comes with a download manager too.
Supports pause/resume.Just right click on the source link from where the file has to be downloaded and start the download with DownThemAll.

Fission: Shows a progress bar in your address bar, yes yes they copied it from Apple Safari. Quite cool looking.

Webmail Notifier:
One of the best add-ons, manage all your email accounts from one place. Now you dont even have to login to your account to check if a new mail has arrived. Install this, and disable auto-login for webmail notifier at the add-on's screen.

Enter your email address and password for any email service, you can enter multiple addresses for each mail service, but only one in each mail service can be selected as default. Eg: If you have 2 yahoo accounts, 1 hotmail account and 3 gmail accounts, only 1 can be default for gmail and yahoo.
This little add-on sits on the status bar and notifies you with a small pop up when a mail arrives

Better Privacy: Deletes flash cookies, and non deletable cookies from sites like youtube or ebay, Google. Very useful to keep your tracks clean.