With an engine fire upfront, whats the chances of it burning the mounts and causing the engine to seperate from the airframe?
Without doing the sums, I'd say pretty remote. You've got engine mounts - normally made of steel that are deliberately over-engineered to take over 6 times the weight of the engine downwards, so those would have to degrade a great deal to cause a failure in gliding flight. You might lose the engine in the rather higher loads of a heavy landing subsequently - but that's a separate and rather less serious problem.
The firewall is probably much less robust than the mounts, but if that's gone, probably so are you I'm afraid - but I've never heard of one getting burned through before ground impact so I'd not lose sleep about it.
G