With noscript, browsing still works fine, you just have to explicitly allow the javascript you want/need rather than allowing just any site to execute code in your browser.
No, even when you do allow a site to run JS, NoScript includes additional XSS, XSRF, and "click-jacking" protections that aren't normally offered by Firefox.
noscript is per-source, so you can whitelist their <script> blocks and jquery.js, but that random javascript in an onmouseover in a forum comment will do nothing.
Maybe it's been a while, but I thought that NoScript was per-domain. In the event of a XSS, the javascript maybe included from the page's domain. NoScript wouldn't help you here. IIRC, NoScript wouldn't say, "Hey this script wasn't here the last time you visited this domain, do you want to allow it?"
