There is an odd bug in some versions of FireFox which causes problems with validating dates. It is a result of a particular combination of versions of FireFox and the MooTools library used by Joomla!.
User tillar1 posted the following fixes either of which will work OK (the first is to upgrade MooTools):
Well it appears I fixed the problem. As per advice above I researched the problem and discovered that the Mootools library version I have is no longer supported in Firefox 18. I have 1.2 and I believe 1.26 fixes the issues for us legacy users.
But instead of upgrading the Mootools libray, I simply went into the following files:
\www\components\com_chronocontact\libraries\includes\JSvalidation.php
\www\components\com_chronocontact\js\jsvalidation2.js
\www\components\com_chronocontact\js\mooValidation.js
And renamed every instance of:
validate-alphanumber to validate-anum
&
validate-alpha to validate-alp
Done!
