NoScript's unique whitelist based pre-emptive script blocking approach prevents exploitation of security vulnerabilities (known and even not known yet!) with no loss of functionality...
Operating NoScript is really simple.
When you browse a site containing blocked scripts a notification, similar to those issued by popup blocker, is shown.
For each site you can decide to allow the exact address, or the exact domain, or a parent domain. If you enable a domain (e.g. mozilla.org), you're implicitly enabling all its subdomains (e.g. www.mozilla.org, addons.mozilla.org and so on) with every possible protocol (e.g. http and https). If you enable an address (protocol://host, e.g. http://www.mozilla.org, you're enabling its subdirectories (e.g. http://www.mozilla.org/firefox and http://www.mozilla.org/thunderbird), but not its domain ancestors nor its siblings, i.e. mozilla.org and addons.mozilla.org will not be automatically enabled.
By default only the 2nd level (base) domain is shown (e.g. mozilla.org) is shown in the menus, but you can configure appearance to show full domains and full addresses as well.
Java, Silverlight, Flash and other plugins
On a non-whitelisted site you can still temporarily allow an individual plugin object with just one left click on its placeholder (screenshot). The movie/applet/clip will stay enabled until the end of the session or until you Revoke Temporary Permissions.
Middle clicking on a Java/Silverlight/Flash/Plugin object placeholder opens it in a window of its own.
Right clicking on a Java/Silverlight/Flash/Plugin object placeholder opens the context menu for links, allowing you to save the content with Save Link As....
Holding down the Shift key and clicking on a Java/Silverlight/Flash/Plugin object placeholder temporarily hides it.
You can also use the Blocked Objects menu to find out which plugin content instances you're blocking even if their placeholder is not easily visible, and/or enable them individually, per site or per type.
Some sites, especially those serving ads, can appear in your "Allow ..." menu more often than you like, making it too much long and noisy.
If you know you don't want to allow a certain site now and in the foreseeable future, you can permanently mark it as untrusted: just click the NoScript icon, open the Untrusted menu and select the Mark bad-site.com as Untrusted menu item.
NoScript won't even propose you to allow it again and your NoScript will be even more clean and usable.
If you later change your mind, don't worry: just open the Untrusted menu again (on the same page), and you'll find the Allow bad-site.com command there.
This feature is especially useful if you decided to use the (not recommended) NoScript Options|General|Temporarily allow top level sites by default mode, because sites marked as untrusted won't be allowed anyway.
Advanced users: even though the untrusted sites blacklist has no listing UI of its own, you can mass-edit it either modifying the noscript.untrusted about:config preference or using the Import/Export functionality of the NoScript Options|Whitelist panel, knowing that the untrusted entries are exported under an [UNTRUSTED] header.
Cross-Site Scripting (XSS) vulnerabilities are usually programming errors made by web developers, which allow an attacker to inject his own malicious code from a certain site into a different site. They can be used, for instance, to steal your authentication credentials and, more in general, to impersonate you on the victim site (e.g. your online banking or your web mail).
NoScript XSS notification and its menu NoScript features unique Anti-XSS counter-measures against XSS Type 0 (DOM based) and XSS Type 1 (Reflective, absolutely the most common) attacks targeted to whitelisted sites.
Changelog for this release:
Fixed live bookmarks in Firefox 48 or above