No Script is an addon that improves the security of Firefox by allowing only trusted scripts (from a whitelist) to execute. This is particularly helpful when browsing bank websites, ecommerce sites etc. I use Chrome for daily purposes, and typically NoScript in Firefox only for all my online transactions that involve disclosing personal data like credit card number, bank account login details etc.
The NoScript status bar displays information about which objects (for example, advertisements and pop-up messages) and scripts are currently prevented from executing themselves on your system. The following two figures are prime examples of NoScript at work: In Figure 2, NoScript has successfully blocked an advertisement created in Adobe Flash Player on a commercial website.
Since scripts from your whitelisted domains run, you will not experience any noticeable loss of functionality. Enabling scripts on trusted sites can be done with a click on the NoScript status bar icon. When NoScript detects scripts, it will block them and show itself on the status bar.
NoScript also protects you against the dangerous XSS vulnerability (cross-site scripting) and clickjacking attacks. A cross site script permits hackers to run harmful code. Clickjacking is when the script emulates a user click, allowing script execution or unwanted actions to occur.