Removing unwanted content on webpages
From MozillaZine Knowledge Base
Many webpages contain advertising or other clutter you'd rather not see. These extensions can help you control unwanted content.
Adblock and Adblock Plus
Blocking with a Hosts File vs. blocking with Adblock Plus
The Hosts file contains the mappings of IP addresses to host names. You can use a HOSTS file to block ads, banners, 3rd party Cookies, 3rd party page counters, web bugs, and even most hijackers. This is accomplished by blocking the connection(s) that supplies these little gems. It should be your first line of defense. See Blocking Unwanted Parasites with a Hosts File
The hosts file is not an extension and will work across your entire systems regardless of what browser or what Firefox profile you use. But the entries are not very generic. www.example.com, example.com, my.example.com would each need separate entries. Adblock Plus could block all of those and more with a generic entry blocking example.com altogether, but it only applies to your Firefox usage in the profile(s) it is installed in.
Adblock Plus is easily turned on/off with a toolbar button on a toolbar, the hosts file is not easily turned on/off (must be removed/renamed by an Administrator) and the entries more restricted to absolutes in blocking (everybody) as opposed to those where users many want to make choices to block or not by adding/removing a filter or making their own exception to a filter within a filter subscription list. Adblock Plus maintains each subscription list, and your own filters separately, so you can update a subscription without having to redo all of your own filter work. You can bring up the "Open blockable items list" ("Ctrl+Shift+V") on a web page to see what ADP has blocked, the actual filter blocking and the via the list's context menu determine the filter source (actual subscription involved).
ADP has a reporting mechanism in their options to report sites so they will be blocked.
Protection in Firefox
Firefox has a reporting system for reporting Web Forgeries (Phishing) under the HELP menu, maintained by Google and used by all browsers, but you must have checking turned on in your Firefox options.
Tools > Options > Security
Other extensions that can remove/modify unwanted webpage content include: