Downloading Youtube videos played in Firefox - Is it legal?

First of all, how to copy a youtube video you just played in Firefox :
Source : Experts-exchange

"...
If you use Firefox, you can grab FLV files out of the cache.

In Windows XP, the Firefox cache is usually here:
C:\Documents and Settings\[your windows user name]\Local Settings\Application Data\Mozilla\Firefox\Profiles\[some number.default]\Cache

In the cache folder, switch to 'details' view. Once you play the video, a large file will appear at the top of your cache folder (you may have to refresh the view). Add the .flv extension and you're good to go. This should work with any site that plays video through Flash, including YouTube.

..."

But, is it legal?
No.
Source : Youtube terms
"...
Content on the Website is provided to you AS IS for your information and personal use only and may not be downloaded, copied, reproduced, distributed, transmitted, broadcast, displayed, sold, licensed, or otherwise exploited for any other purposes whatsoever without the prior written consent of the respective owners.
..."

Also See :

YouTube Threatens Legal Action Against Host of Video Download Tool