Can "fire together, wire together" be undone? Are new connections continuously forged in the adult human brain, or do we just start all wired together and then prune until we have something we like?
The phrase "fire together, wire together" comes from an explanation of Hebbian Learning and refers to the adaptation of synapses as a response to the firing of already connected neurons. This is one of several Synaptic Plasticity mechanisms. Two others that exist are Long Term Potentiation (strengthening and creation) and Long Term Depression (weakening and destruction).

All of these mechanisms are still being researched heavily to understand their detail better. However, I hope this gives you a good starting point.