Firefox (seem all versions) has an issue with the Ajax validation and WW hidden tag.
If there is an <ww:hidden .../> in jsp view, when onblur event occur more one times new error message is added without remove the previous message. This issue occur only when there is the hidden tag.
 
For example:

This is JSP file

<ww:form id="f1" action="Login" method="post" validate="true" theme="ajax" >
<ww:hidden name="username"/>
<ww:textfield label="UserName" name="docente.userName" required="true" />
<ww:password label="Password" name="docente.pass" required="true"/>
<ww:submit id="b1" value="Accedi" />
</ww:form>

and this the *-validation.xml

<validators>
<field name="docente.userName">
<field-validator type="requiredstring">
<message> Nome utente non inserito </message>
</field-validator>
</field>
<field name="docente.pass">
<field-validator type="requiredstring">
<message> Password non inserita </message>
</field-validator>
</field>
</validators>

If i click more times, obviously without insert data, on UserName textfield or Password textfield, the error messages are displayed without remove the previouses message