I did a quick google on this so I'll not duplicate other findings.
In earlier versions of Firefox, like other browsers, there was an X button you could click to make a particular page simply stop whatever it was doing, no more loading etc. If there is one on the current version, Firefox 18, then I can't find it.
So, how do I make a webpage stop? This is for pages that constant load video clips, movie or game trailers, etc... when I don't want to close the tab, just halt it to come back to later.