Problem
You want to use automated proxy detection through DHCP Option 252. Chrome, InternetExplorer and Edge work flawlessly but Firefox ignores settings.
Solution
DHCP WPAD configuration will be supported only from Firefox version 63 on (see here).
As a workaround you might use DNS based proxy configuration. To do so create an alias record “wpad” in DNS to point to the UCS Proxy hostname (ie “ucs2”).