I've searched around and found little on the subject of firefighting. I'm actually quite surprised it wasn't at least mentioned in Elminster's Forgotten Realms, unless I missed it.
I've found that Waterdeep has a guild whose duties include watching for fires and fighting fires, and I assume most metropolises have similar guilds. I've found mention of water brigades in some cities too. For places that do not have either of these, would it rely on just whoever wants to help? Is having water nearby common in towns? Would dirt and sand be kept nearby for firefighting purposes? Would wands or scrolls of water elemental summoning be something that cities may utilize?
I assume the answers are fairly common sense, but I was curious about what other people may know that I've missed.
I know that Ed has mentioned that rooftop cisterns that are used to supply drinking water are also used for fighting fires (or, if the building becomes too damaged, the collapse of the cistern will extinguish the remaining fire).
For the Guardian Wizards, their charter says that part of their duties is to fight fires (it was one of the reasons why the organization was created). This is done with wands of fire extinguishing and spells such as affect normal fires. There are wizards posted around town at all times to respond to fires. For a fee, the owner can hire the Guardian Wizards to cast the spell unburn to restore items that were destroyed in the fire. If the fire's origin was of a suspicious nature, an investigation may be required to be completed before any unburn is cast or the owner may get a free one if the unburn is necessary to the investigation.
"Iriaebor is a fine city. So what if you can have violence between merchant groups break out at any moment. Not every city can offer dinner AND a show."