Hrm. I'll install FireFox 2 on another PC in a bit and check -- the fact it's so inconsistent is making me lean towards the problem being end-user related, such as an Extension or AddOn causing a conflict.
I don't know if it's possible, but what would
really help is if you could get the 'View Source' output from one of the users experiencing the problem.
If I haven't said it before, Tralala, thanks for the help you've provided so far. You've been integral in getting things where they are now.