[@nightgunner5 Member 16 years, 9 months ago]

The problem is quite simple, really.

<li<?php if ($current_forum_id=get_forum_id()) {echo " class='on'";} ?>> just needs to be changed to <li<?php if ($current_forum_id==get_forum_id()) {echo " class='on'";} ?>>

(Can you spot the difference?)