Someone signing off with a first name is not an invitation to refer to them by such. I'll admit the judiciary is probably one of the only workplaces where titles really matter, but judges routinely sign off internal emails (to associates, support staff etc) with their first name, but they're always 'judge' or 'his/her honour'.

It's all contextual, of course. There's no hard and fast rules - all depends on whether you've met before; whether they're your peer/superior; if it is work related/personal; what profession they belong to; whether they hold public office; whether you're asking a favour; whether there is any likelihood your email will end up annexed to an affidavit or splashed across a newspaper page; whether you know them through their work or as another parent at the junior footy club. If in doubt, erring on the side of formality is usually safer, particularly if they're clearly a superior to you (eg student / lecturer).