Fire extinguishers were devices used to put out fires. In some situations they could also be used as makeshift weapons.
Early evidence of fire extinguishers on board the TARDIS was seen when Nyssa had to contain the fire caused by her sonic destruction of the Terileptil android. A small model usable by a single hand was kept in a wicker basket just outside her bedroom. (TV: The Visitation)
The Eighth Doctor later used a fire extinguisher in the TARDIS library when a page from his copy of The Strand caught on fire, (PROSE: The Bodysnatchers) and the Tenth (TV: The Runaway Bride) and Eleventh Doctors (TV: The Angels Take Manhattan) sprayed an extinguisher into the console room after particularly rough landings.
The Third Doctor's UNIT laboratory was also equipped with fire extinguishers. Upon her first meeting with the Doctor, Jo Grant used one when she saw smoke emanating from the Lamadine steady state micro welding the Doctor was performing on the TARDIS dematerialisation circuit. The Doctor berated her for this as she had ruined his work. (TV: Terror of the Autons)
The Doctor invented a fire extinguisher which produced eighty times more foam than normal extinguishers. He used it to put out a fire from a Bunsen burner on the TARDIS console. (COMIC: Bad Vibrations)
Fire extinguishers could be used more offensively, however.
On a parallel Earth, the Third Doctor, Greg Sutton and the Alastair Lethbridge-Stewart used fire extinguishers against the Primords. When the Doctor returned to his own universe, he used a fire extinguisher against the Primord John Bromley, killing him. He and Greg Sutton later used fire extinguishers against the mutated Professor Eric Stahlman, killing him. (TV: Inferno)
In 1968, Edmund Trevithick used a fire extinguisher to spray an insectoid creature and to beat it with it. He later shot the fire extinguisher, causing it to explode and kill the creature. (PROSE: Nightshade)
On Világ, the Sixth Doctor and Justice Rossiter used fire extinguishers against hospital patients who had been given transfusions of Killoran DNA. (AUDIO: Thicker Than Water) The Sixth Doctor used a fire extinguisher to break the window of a steam room to rescue Peri, who then used it to cool herself off. (AUDIO: Year of the Pig)
Other species were perhaps more susceptible to the effects of fire extinguishers. Clockwork Droids, (TV: The Girl in the Fireplace) Silurians, (TV: The Hungry Earth) and people like Harvey who had been converted into a Dalek puppet by the nanocloud (TV: Asylum of the Daleks) could all be rendered inactive for several hours by the use of an ordinary fire extinguisher.
Sometimes, however, fire extinguishers proved ineffective weapons. Sarah Jane Smith first sprayed Odd Bob the Clown with a fire extinguisher, then hit him with it, but neither action had any effect. (TV: The Day of the Clown)
In some situations, extinguishers were used more as projectiles – objects to be simply lobbed at a problem. When Brian Hughes locked the doors of the Pinehill Crest Hotel, for instance, Ace tried throwing a fire extinguisher to break them, only to find that they were made of safety glass. (AUDIO: The Shadow of the Scourge)