[kh99]

Do you want to allow ?b=20409 in addition to t? You could edit includes/class_core.php, and around line 1500 make this change (add the part in red):

	/**
	* Translation table for short name to long name
	*
	* @var    array
	*/
	var $shortvars = array(
		'f'     => 'forumid',
		't'     => 'threadid',
		'b'     => 'threadid',
		'p'     => 'postid',
		'u'     => 'userid',

If you add the 'b' line after the 't' line, if they happen to both be present the 't' value will be used. If you'd rather have the 'b' value used in that situation, you can switch the order of those lines.

If you want to replace all 't' with 'b', you'd have to find everywhere a thread link is created and change it, then change 't' to 'b' instead of adding a line (but if you already have a site running you probably don't want to do that, you'll break all links to threads, including from search engines).