A fire hazard was anything that had the potential to cause a fire or make it more dangerous.
In 2372, Quark asked Odo to remove several striking employees from the front door of Quark's, saying they were probably a fire hazard. Odo agreed with him, but was unable to do anything about it as Captain Benjamin Sisko had ordered him not to interfere with the employees' freedom of expression. (DS9: "Bar Association")
In 2376, Tuvok blew out a birthday candle on a birthday cake Kathryn Janeway presented him with, after she requested he do so despite his protesting that it was not Vulcan tradition. To save face, he claimed that the candle was a fire hazard, if left burning. (VOY: "Fury")