IE Tab
Allows you to make a list of websites that you want to be displayed using IE settings. For example, some sites are only viewable with IE. This addon allows you to visit those types of websites using Firefox.

Fasterfox
Optimizes prefetching, simultaneous connections, pipelining, cache, DNS cache, initial paint delay and has a built-in pop-up blocker.

SafeCache
Defends against web privacy attacks that remote sites can use to determine your browser history at other sites.

SafeHistory
Defends against web privacy attacks that remote sites can use to determine your browser history at other sites.

AdBlock Plus
Right-click on a banner and choose "Adblock" from the context menu - the banner won't be downloaded again.

KeyScrambler
Encrypts your keystrokes at the kernel driver level to protect your login information from keyloggers.

SafeDownload
Automatically scan downloaded files for viruses. You can choose to scan immediately after the file has downloaded or wait until Firefox exits. Works well with NOD32 anti-virus.

Clean and Close
Replaces the Clean Up button in your Download Manager window with a Clean And Close button that does just that.

McAfee SiteAdvisor
Places an icon next to search results to let you know if the site is trustworthy, a scam, dangerous to your computer if the site has not yet received a diagnosis.

One has to be careful with FasterFox because of the way it works, it can be considered as being incredibly anti-social due to it adding extra load onto some web servers.