It's not a Firefox issue, it happens in all browsers. What modifications have you installed? Give me a list of anything that isn't stock, and their page on vB.org.
This is a vBulletin issue, and every thread ever created about it, points to bad modifications or an issue in a template. I can ask a very well known vB modder or vB/vBSol. developer if they can take a look at the issue for you guys, if you'd like that.
Sound fair?
But mind taking a look in the footer template? I turned back on DST correct 2 nights ago, and the errors stopped then. Perhaps another administrator took a look and fixed it, which is why you're not seeing any missing code.