Hi, welcome to TSF
This is our current guide to general security software
PC Safety and Security--What Do I Need?
More specific to Firefox, I would advise installing the following plugins:
NoScript (blocks java, flash, etc on a site-by-site basis, with an icon in the status bar for additional options)
CustomizeGoogle (loads of features for stopping google snooping on your searches and web history, removing adverts, etc)
McAfee Site Advisor (icon in status bar, red for potentially dangerous, green for ok)