Please do use lightboxes _if_ your site will not work on that browser. A perfect example of when you want to interrupt the the natural flow! I've found ie6-upgrade-warning works well- http://code.google.com/p/ie6-upgrade-warning/ for what you might be trying to do.

Most importantly, look at how many of your users are using IE6, or anything other than FF. Will the better experience for your FF (or simply IE6+) users outweigh losing almost all of your IE6 users? Keep in mind that many people who use IE6 now simply can't upgrade because of hardware or administrative constraints.