The extension Adblock Plus for Firefox is the best and most flexible ad blocker available. With the existing filter lists it can block almost all advertisements on the web.
Xmarks is the #2 bookmarking add-on. Keep your bookmarks, passwords and open tabs backed up and synchronized across computers and browsers. Search smarter with website ratings and reviews displayed with your search results.
Bookmarking
NoScript for Firefox pre-emptively blocks malicious scripts and allows JavaScript, Java and other potentially dangerous content only from sites you trust. Download it now for free! - what is it?
WOT is easily gamed/spammed. "ratings" are nothing more than users' opinions who are often trying to "black list" sites for nothing more than having a view different from their own. The addon offers NO security whatsoever, just users' opinions that are often very misleading.
This is a must-have add-on for safe surfing. I wouldn't be without it! It's been on Mozilla's recommended list for a long time, plus I've seen it written up on BNET, ZDNet and the computer magazines. It's very trustworthy.
Vital Extension: WOT scans websites for phising schemes, adware, and basic malicious content and displays the results before you visit a questionable site. Don't browse without it!