For the life of me, I couldn't find which file it was in.
libs/html.inc.php @ about line 562 of UBBT 7.5.8-r2
And while you're in there, if you wanted to change the refresh delay for older browsers; about 3 lines up, change "$delay = 2;" from 2, to 5, or whatever. The number is the automatic page refresh delay in seconds.
BTW, I just updated the OP to add a better description of what this mod does.