Securing the Firefox web browser# 1 - Install
Adblock Plus - This addon blocks most obtrusive adverts, including annoying talking smiley adverts.
# 2 - Install
No Script - This utility blocks javascript from websites which you don't click on the No Script box at the bottom and setting it to 'allow', but its recomended to disable XSS filtering if you're doing purchases online as this has caused me issues before.
# 3 - Install
Ghostery - This addon blocks and alerts you to website analytics software. Click the ghostery icon on the status bar at the bottom, and click Options, go to the 'Blocking Tab'. Tick the 'On' for website bug blocking, and then press the underlined 'All'. Then click OK.

# 4 - Install
Spybot S&D, and use the immunisation feature.
# 5 - [Advanced Step] Consider using
PrivoxyI'll be including screenshots to help make setting up these addons is a bit more simple.
Needless to say, Anti-Virus and a firewall are necessities these days.
Anti Virus:
AVG Free (free for home use)
Anti Virus:
NOD32 (non-free)
Anti Virus & Firewall:
Comodo Internet Security (free for all uses without paid for upgrades)
I personally use Comodo Internet Security, but NOD32 from ESET is an option i would consider if you've got some spare cash.