1. Cool Preview
Preview links and images without leaving your current page or tab. Just mouseover any link, and a separate preview window appears to show you the content.
Download link:-https://addons.mozilla.org/en-US/firefox/addon/2207/




2.  Fastest Fox
Save time by speeding up some tasks like... auto-load the next page, automatically show definitions of select text ,auto copy selected text,  improve searching, and more
Download link:- https://addons.mozilla.org/en-US/firefox/addon/9825/




3. Read it Later 
Save pages to read later with just one click. When you have time, access your reading list from any computer or phone, even without an internet connection!
Download link:-https://addons.mozilla.org/en-US/firefox/addon/7661/




4. Skip screen
No need to wait for countdowns on sites like Rapidshare, Megaupload, zShare, Mediafire, and more!!!
Skip Screen will start download by self for you.:)
Download link:-https://addons.mozilla.org/en-US/firefox/addon/11243/






6. Web Mail Notifier
WebMail Notifier checks your webmail accounts and notifies the number of unread emails...
Supports : gmail, yahoo, hotmail, aol, daum, naver, nate, paran, POP3/IMAP and more
Download link:-https://addons.mozilla.org/en-US/firefox/addon/4490/




7. gTranslator
Google translate gadget for firefox. This extension translates entire webpages into a language of your choice with one click.
Download link:-https://addons.mozilla.org/en-US/firefox/addon/76233/