These Firefox add-ons block "advertising opportunity detection" elements that are not usually blocked by Firefox:
Several Firefox add-ons of interest:
AdBlock Plus ... allows the sneakier counters and trackers to watch you. Add it on and you will be asked which "list" to subscribe to. I was advised to use "Easy List". It is a list of better-known adware trackers. When running, for me, it is a bar at the bottom of the browser, which shows something like "1 / 3" blocked. Yuo can then block or allow by mousing over that bar.
Ghostery ... makes you "invivble" to tracking you choose to not be tracked by. When someone like "DoubleClick" cn see you, a box in the upper right part of the browser will show the site in bright white letters. When you choose to block it, it becomes grayed with a line through it to show it is blocked.
Better Privacy ... Flash players like to leave cookies in its "application space". This space is beyond Firefox control, so these cookies stay behind. BetterPrivay will remove these "other" cookies and tell you it did so when you exit Firefox.
HTTPS Everywhere ... forces Firefox to first try an https connection to a site when in publicinternet spaces. It will default to http is no https is available.
NoScript ... blocks Java/other scrits in browsing. A bit annoying, as it tends to block first, then you must allow. But not too annoying.
Tracker Block ... blocks tracking. Do not se much interface for it yet, but I might yet.
Click&Clean ... no sure what it does yet but will figure it out.
WOT ... who to trust. Gives a status box in the upper left part of the bar telling you if a site you are in is not trustworthy.
Flagfox ... shows a flag to show what country the server is you are using.
Certificate Patrol ... tells you what certificates are loaded, allowing you to block if th site will still work.
Also ... go to http://winhelp2002.mvps.org/host.txt
This is a "hosts" file that lists knows virus sites. If you system tries to access one of those, this hosts file redirects the site address to "home" instead of the target. Re-download every few months for a new list of 15000+ sites that are bad.