Userscripts.org

Great scripts. It would be even nicer if the default save-as filename is some unique string such as the youtube video id.

I think Mindeye already explained that it wasn't possible : the name of the video is decided by the server, not the client, so it can't be changed with Javascript. All videos are stored with the filename "video.mp4" or "video.flv" on YouTube's server.
As for changing the name when the "Save as" window pops up, I don't think Javascript can do that either because it's a part of the browser, not the webpage.

Could be wrong though!

I believe only extensions could do that. Because they're closer to the browser.

Couldn't it just grab the Title that is above the movie and
use this one as the prefered filename ?

There's also the problem of unsupported characters for filenames (like slashes), I could assume they could be replaced with a space.

Is it not possible to parse the title above the video and use that?
I am sure there are scripts (I may be wrong here though!) that take info from a page in a similar way.
Hmm I need to find one and have a look in it maybe.
I have an addon that does this for downloading youtube videos but I want to ditch any unnecessary addons in favour of this script if I can!
Please can someone maybe give me a clue on the code to take the title of the video and use it in this way?

The problem isn't to parse the title of the video, that would be too easy. The problem is that you then need to tell the browser "Use this string as the filename when saving the file". However, a Javascript script cannot do this, maybe because of security issues.
As you said, only add-ons for Firefox can directly tell the browser a filename, because they are closer to it. Now maybe some script writers found a workaround for this but I have yet to find one that is able to do that.