How do I block ads in Firefox

Ghostery

is one very useful extension that can be used to block ads, trackers etc. This is done via its easy to use interface:

The two-position switch can used to enable/disable various each entry, whilst the arrow button can be used to allow this. Sites can be whitelisted, so that the trackers only on those sites. Many settings are available in options panel (the URI for it is resource://firefox-at-ghostery-dot-com/ghostery/data/options.html), which allows the changing of various settings. Most of these settings are shown when you first add Ghostery, and can be used to set it up.

A paranoid more effective appraoch to block everything is to disable GhostRank (sends data to Ghostery^???), block all trackers and cookies (the ones needed for some sites to work can be unblocked easily later when needed), and enable 'Block new elements by default'.

Ghostery should also block things from using cookies - Flash/Silverlight cookies can also be removed if enabled in settings.

^???It is worth noting that Ghostery is supported by Evidon, a ad company that sells the data from GhostRank. Those that don't want this may want to make sure GhostRank is disabled

NoScript Security Suite

This is a much simpler, and in someways more effective addon, as it blocks JavaScript and other features unless told not to. This is a rather effective way to prevent ads and trackers from loading.

The problem with this addon is that it is so effective, many sites won't work unless it is told not to block them. This can be done by 'allowing' (or temporarily allowing for this session) the site's domain (and others it needs - e.g. askubuntu.com needs sstatic.net, stackexchange.com and ajax.googleapis.com). You can also set what to block in the Options:

For instance, @font-face is now used by many sites and can mostly be trusted, so that can be unblocked (it is blocked by default). Also Video/Audio need to be enabled if you want to watch HTMl5 videos, etc.

N.B. I have found on systems the menu seems to open with the shortcut Ctrl+Shift+S - handy if you can't be bothered to move the mouse.

Both of these addons work very well together, and can be used stop nearly every single annoying ad and tracker in site. Both also integrate fairly well into Firefox 30's new menu, meaning there are less icons to clutter the panel

                                                                   
                                                  ^{Also note the stupidly large amount of trackers Ghostery is blocking...}

Other addons are also available, including Adblock Plus.