Firefox plugins which should be installed:

Ad block plus: https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/; Helps in blocking adds. It also supports adding custom filters and option for exporting / importing filters.

Ghostery: http://www.ghostery.com/; It prevents well known tracking websites from tracking user activities and co-relating different actions performed by same user on various websites.

Xmarks: http://download.xmarks.cNautilusom/download; It allows synchronization of bookmarks. Newer versions also support synchronization of passwords and tabs. We can open site http://www.xmarks.com/ from anywhere to access our bookmarks on other machines.

Better privacy: https://addons.mozilla.org/en-US/firefox/addon/betterprivacy/; Better privacy helps in clearing cookies saved by various sites that can be used to recognize same user.

Web of trust (WOT): https://addons.mozilla.org/en-US/firefox/addon/wot-safe-browsing-tool/; It helps in giving reputation information about domains. We can also give our inputs for good / bad domains.

NoScript: https://addons.mozilla.org/en-US/firefox/addon/noscript/; It helps in blocking javascript by default on all websites. We can enable scripts temporarily for a page or for a domain. We can also create permanent exceptions. It also disables scripts from internet pages from accessing intranet content (ABE).; Considerable patience and practice is required to use NoScript. But privacy has a price. Noscript is also capable of saving configuration in form of bookmark. Hence using NoScript with XMarks allows synchronization of NoScript rules across machines seemlessly.

Refcontorl: https://addons.mozilla.org/en-US/firefox/addon/refcontrol/; It helps in changing HTTP referrer value sent to many websites which help in discovering from where we clicked link to visit particular website. It disables tracking of how we go from one website to another.; This would cause banking websites which are paranoid (as they should be) to stop working. Hence exception should be created for bank websites to work properly.

Disconnect: https://disconnect.me/; Similar to ghostery. Installed for additional protection against giants like google, facebook and twitter

RequestPolicy: https://addons.mozilla.org/en/firefox/addon/requestpolicy/; Protects from Cross Site Request Forgery (CSRF) attacks

CsFire: https://addons.mozilla.org/en-us/firefox/addon/csfire/; Protects from Cross Site Request Forgery (CSRF) attacks

HTTPS Everywhere: https://www.eff.org/https-everywhere; Converts normal requests to HTTPS requests using complex rulesets. It is always not as simple as changing protocol, as sometimes URL needs to be changed. For example http://en.wikipedia.org/wiki/World_Wide_Web was for a while changed to https://secure.wikimedia.org/wikipedia/en/wiki/World_Wide_Web

HTTPS Finder: https://addons.mozilla.org/en-us/firefox/addon/https-finder/; Searches for HTTPS option on browsed website and adds appropriate rules to HTTPS Everywhere so that rules do not need to be added manually.

Plugins not related to security

Add to search bar: https://addons.mozilla.org/en-US/firefox/addon/add-to-search-bar/; Allows to add search box option of any website to search toolbox. Can be used to create search for imdb, notes_wiki, etc.

Imdb search: https://addons.mozilla.org/en-US/firefox/addon/imdb/?src=search; Allows searching at imdb directly from search toolbar

Duck duck go: https://addons.mozilla.org/en-US/firefox/addon/duckduckgo-for-firefox/; Allows to search via duck duck go for enhanced privacy

Other useful plugins

Tamperdata: https://addons.mozilla.org/en-US/firefox/addon/tamper-data/; This allow modifying HTTP post values and other heads for complete control on requests that go to web server.

fillform: https://addons.mozilla.org/en-US/firefox/addon/fillforms/; This allows saving a form values using Alt+W and then restore saved values using Alt+Q

Tab session manager: https://addons.mozilla.org/en-US/firefox/addon/tab-session-manager/; This allows saving tabs for future reference

Also refer to Disable use of iframes in firefox for securing firefox from iFrame based Clickjacking attacks.