The best add-ons i've ever used with my firefox are :
1)Adblock Plus:
Removes all ads on most websites and blocks flash/java objects
Download link : http://adblockplus.org/en/
2)Disconnect :
Stop third parties and search engines from tracking
the webpages you go to and searches you do.
-disables third-party tracking,
-truly depersonalizes searches,
-shows blocked resources and cookies,
-lets you easily unblock services, and
*Chrome, Firefox, and Safari supported
(3ayne it removes those facebook & other sites like buttons and comments etc..)
Download link : http://disconnect.me/
3)Redirect Remover:
This extension will removes Redirects like http://redirectremover.mozdev.org/?www.mozilla.org from Links and Images removes annoying random links when you click on some links
Download link : https://addons.mozilla.org/en-US/firefox/downloads/latest/537/addon-537-latest.xpi
4)TrackerBlock:
Complete control over online tracking using multiple methods, including cookie blocking, persistent opt-out cookies, Flash and HTML5 control, and Do Not Track signals. Visit us to learn more
Download link : https://addons.mozilla.org/firefox/downloads/latest/247581/addon-247581-latest.xpi?src=dp-btn-primary
5)Status-4-Evar
Restores back the good old status bar in the bottom with extra features
Download link :https://addons.mozilla.org/firefox/downloads/latest/235283/addon-235283-latest.xpi