When the DEU was introduced in 1984, the decision was made that not all trades would have all uniforms. In most cases, the rationale was obvious: boatswains would wear navy DEU, infantry would wear army DEU, pilots would wear leisure suits air force DEU, etc. The "purple" (i.e. mostly hard-core logistics) would wear all three. That left a small number of question mark trades where decisions were often made based on what the previous three services had done. For example, neither the RCN nor the RCAF had a dental branch in 1968 as all dental services for the RCN, Canadian Army and RCAF were provided by the Royal Canadian Dental Corps (part of the Army); therefore, all dental personnel now wear army DEU. In the case of firefighters (and Meteorological Technicians) , although both the RCN and RCAF had firefighters (and Met Techs), their main purpose in both services was to fight aircraft fires (or forecast weather for flying operations). Ships that did not carry aircraft, did not carry firefighters either (and only carried junior Met Techs). Thus, because firefighters' (and Met Techs') main purpose in life is in support of flying operations, the decision was made to put them all in air force DEU.
Having said that, I am aware of one firefighter and one Airfield (?) Engineering (who was the CF Fire Marshal at one point) officer who wore navy DEU. In the case of the firefighter, he was a crusty old salt who had actually served as a firefighter in the RCN and claimed vested rights (I knew a Met Tech in a similar situation who lost that battle). The officer asked for and got permission to change DEU because of where she had served throughout her career. Normally, she would have worn air force DEU.
