I got this working by modifying the web.config.  As it turns out, someone had &lt;xhtmlConformance mode="Legacy"/&gt; in there, which is an older setting (the current default is "transitional").  Thanks for everyone's help.

ASP.Net WebForms requiredfieldvalidator not working in FireFox?
Some people use brainstorming, others use mindstorming. I could not find the difference between the two words. 
