I just had to sign up for someone (sillyzombie; Firefox user) because his e-mail address didn’t fit in the field on the form. I have no idea what could cause that, but you might want to have a look at it.
(I could use this opportunity to extol Opera’ superiority over Firefox, but I’ll refrain since that’s already obvious and this particular instance might not be Firefox’s fault ;-))
The problem was, you confused the Username and Email fields and tried to enter your email into the Username field. I heard this from other users as well.
I guess the problem is that people expect the Username field first, and then the Email field, which is not the case with the Xoops registration form. Too bad that it’s not skinnable. I need to find a way to hack the table width, so the fields are not that far away from their labels…