Do aircraft always have the right of way on the ground? Or must they yield to police/fire/ambulance?
Just because one has the right of way, does not mean they must exercise that right. 

If a pilot sees an emergency vehicle, and is able to, I expect he would stop, and either flash his lights or wave visibly to allow an emergency vehicle to proceed.

The aircraft has right of way, however, because the pilot may not see an emergency vehicle, or may not be able to stop quickly and safely.

So, yes, the aircraft always has right-of-way over ground traffic, according to the rules.  They are reasonably expected to yield in certain circumstances.