Check the a4j log and see if it reports any problems, you can also check under the console tab of FireBug to see what is being sent back in the ajax response. FireBug will also show you the content after the page after the ajax update (I think).
If you are still having issues you might get more help at the richfaces forum.
Thanks, no. a4j:form doesn't help. It seems like a general issue with any HTML <form> element being replaced by replaceNode in FF.
I reposted this ro richfaces forum and that folks confirmed this to be a bug in FF