Firefox:

In Firefox the easiest way to control JavaScript is to enable JavaScript by default, then install the NoScript extension which...

"provides extra protection for your Mozilla/Firefox or Flock browser: this extension allows JavaScript and Java execution only for trusted domains of your choice (e.g. your home-banking web site).

This 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."

If you don't have the NoScript extension installed, then you can globally enable/disable JavaScript by going to:

Tools > Options... > Content > Enable JavaScript

Opera:

In Opera you can globally enable/disable JavaScript by going to:

Tools > Preferences... > Advanced > Enable JavaScript

Microsoft Internet Explorer:

In MSIE you can globally enable/disable JavaScript by going to:

Tools > Internet Options... > Security tab
Under the Security tab, select the Internet Web content zone
Under Security Level for this zone:
Click on the Custom Level... button at the bottom of the page
In the Security Settings window scroll down until you get to the section called Scripting (near the bottom)
Under Scripting > Active scripting, select the option marked Enable (or disable)
Click OK