Many websites use scripts.Some of them can be harmful.An attacker can use scripts to perform some attacks such as XSS:
Read more in wikipedia.
No script give you the power to control all scripts and plugins such as flash, java, ...This way you can add trusted websites to the white list and prevent others from running.You will have a more secure browsers this way.
Better performance, better speed and decreasing unwanted bandwidth usage are some side benefits.
In theory you can get viruses on Linux and Mac OS. The reason most people don't is because Linux and Mac OS are not big targets. The malware writers want to cast a wide net with minimal effort. Secondarily Linux/Unix offer more in the way of security and better informed users (in general). That being said I use Flashblock and No Script on Windows, Mac OS X and Ubuntu at all times. Pages load faster, it helps with online anonymity by preventing flash cookies and all manner of other issues. I highly recommend them, regardless of platform. At minimum they make you more aware of what pages are trying to do.
I use NoScript regularly on Firefox, and recommend it for daily use.
It doesn't block ads, so you still support the site costs for their administrators.
However, it does block flash ads, greatly reducing your CPU load when browsing (provided that you have the flash plugin installed)
You can individually allow content to run, so most video sharing sites will start to work after you allow the scripts related to video playback (This may take a bit of guessing if there are several scripts on the page). The permissions you grant may be temporary for the session, or permanent so the site will work unless you decide to block them again.
When filling out forms such as site registrations, it's a good idea to allow the scripts before filling out the forms so you don't have to repeat your work. Allowing a script on a page forces a reload of the page.
The most important protection NoScript grants you is from malicious sites that try to change your window size, post content to social sites or do anything else unwanted. NoScript changes the default action to denied, and you can choose per-site if you judge that the scripts are trustworthy.
Here's the install link for Firefox: https://addons.mozilla.org/en-US/firefox/addon/noscript/