Tower Controller reported ARFF failure to use proper frequency complicated communications with aircraft in distress.

Narrative

Aircraft X was an IFR departure and was cleared for takeoff and switched to Departure. When Aircraft X was approximately 1 mile west of the ZZZ Class D; Departure called and asked me to try Aircraft X again. After reaching out and receiving no response; I asked the OS (Operations Supervisor) to inform Departure that we were unable to establish contact. Aircraft X's data block dropped from the radar display and a few moments later Aircraft X called on LC1 and requested priority handling while entering the downwind at 6;000 [ft]. I asked Aircraft X to state intention and the nature of the emergency. Aircraft X responded they wanted to land RWY XXL and that they had a fire in the cockpit. The OS called Departure to inform them that we had regained contact with Aircraft X and that they were returning to the airport as an emergency. The OS then began notifications for the emergency. I began clearing out my traffic pattern and switching my traffic to LC2. 3 Aircraft were switched to LC2 and Aircraft Y was instructed to make a full stop or exit the airspace. I also instructed [the] VFR Aircraft Z that they may need to hold outside the airspace but could continue inbound for the moment. About this time Aircraft A called on local 1 frequency to request permission to taxi on the movement area to stage for the runway. I instructed Aircraft A to proceed on [Alpha] and pass behind the aircraft taxing [in the] opposite direction.When Aircraft X was established in the downwind; I attempted to give Aircraft X a landing clearance and get additional information. However I received no response. I think the OS attempted to reach out on guard to Aircraft X but there was also no response. I grabbed the light gun and gave Aircraft X a green light to land XXL while Aircraft X was in the base turn. While in the base turn; the OS stated that Aircraft X's landing gear was not down so I made a broadcast advising that the gear appeared up; again with no response. I asked the OS to take over the light gun signals to the aircraft so I could return to scanning the runway. About this point; another controller advised that they observed landing gear on Aircraft X so I made another broadcast to the aircraft with no response.Aircraft X landed and taxied to the ramp under their own power. No injuries and unknown damage to the aircraft.Issues noted: OPS called on local 1 for access to the movement area instead of calling Ground.[Fire Engine] Aircraft Rescue and Fire Fighting (ARFF) staged at the fire station instead of staging at the runway. Ground Control had to prompt [Fire Engine] to reposition.ZZZ Airport OPS should contact Ground Control for access to the movement area. Having to redirect ops to contact ground; or scan the movement area to issue control instructions to ops vehicles distracts controllers working local. OPS should only be on the local control frequency for prolonged periods on runway surfaces or when operationally required.ZZZ ARFF should review; and if necessary; revise their staging locations for emergencies with aircraft reporting inflight fires.