Javascript / Firefox / Java Issue

We have an add that has been working for months, maybe a year. Recently an issue has arisen.

See this code:

function chk_vals() {
			if (ac == "l") {
				jj = confirm("Log Out will lose all data entered. OK?");
				if (! jj) {
					return false;
				}
				document.st.action = "logout.php";
				return true;
			}	
			if (ac == "c") {
				jj = confirm("Cancel will lose all data entered. OK?");
				if (! jj) {
					return false;
				}	
				document.st.action = cancpg;
				return true;
			}	
			if (document.st.pn.value == "" || document.st.padr.value == "" || document.st.city.value == "" || document.st.zip.value == "") {
				alert("Minimum of property name, address, city & zip required.");
				return false;
			}

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Under very specific circumstances, even when there are values in all of the four form fields, pn, paddr, city & zip, the alert occurs.

3 people have tested this, all using FireFox. The problem NEVER occurs for me. It always occurs for person B. For person C, they updated Java on thee computer (all Windows computers), they had been told that Java needed updating. When they did that, it now works.

Is Java used by FireFox in Windows 10 environment? NO Java is used in the web app (all Javascript, php, mySQL, html, Jquery).

Thanks