If yours is a 5 speed, it's more likely a leaking clutch master cylinder than leakin at the firewall.
That said, unbolt the bulkhead connector (1/4" bolt in the middle from the engine bay side, and two gold colored phillips heads on the firewall), pull it out, silicon the sealing edges after you remove any rust from the firewall, and put it back together.
Disconnect the battery just for safety