Instead of having a magnet link open with the default torrent client, is there a way to have the magnet link get copied into the clipboard when it's left clicked?

A script or perhaps an add-on for Firefox to do such thing?

This will override the mouse left click and put it in the clipboard.
You must put // @grant GM_setClipboard in the userscript configuration.
Here is the code!

var x = document.getElementsByTagName("a");
for( var index = 0; index < x.length; index++ )
   if ( x[index].getAttribute("href").indexOf('magnet:') != -1 )
   x[index].addEventListener("click", function(e) {
     GM_setClipboard(x[index].getAttribute("href"), "{ type: 'text', mimetype: 'text/plain'}" );

Good Luck! If you have any more questions let me know.