The NoScript Firefox extension provides extra protection for Firefox, Flock, Seamonkey and others mozilla-based browsers: this free, open source add-on allows JavaScript and Java execution only for trusted domains of your choice (e.g. your home-banking web site).

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...

You can enable JavaScript/Java execution for sites you trust with a simple left-click on the NoScript status bar icon (look at the picture), or using the contextual menu, for easier operation in popup statusbar-less windows.
Watch the "Using NoScript" video kindly contributed by John Wilkerson.