#RewriteCond %{QUERY_STRING} ^t=([0-9]+)&referrerid=[0-9]+$
#RewriteRule ^$ /showthread.php?t=$1 [R=301,L]
#RewriteRule ^showthread\.php\?t=([0-9]+)&referrerid=[0-9]+$ showthread.php?t=$1 [R=301,L]