Well, this has gotta be the quickest that I've ever figured out a problem I posted to any of the BV support forums:

I suddenly realized that I could go into my BV files and widen the page-element box of the menu, extending it as far to the right as the longest piece of text in the sliding menu. So I did, and the sliding items now look A-OK in Firefox (and just as good as before in IE and Safari).

Hope that doesn't sound too confusing. The lesson here is, be sure to make the page-element box containing your (sliding) menu as wide or as deep as the slide-out item that extends the farthest down or across the page. :)