902 nailed it. If you don't pay any attention to the other posts, pay attention to his.
Here's the background on it.
The FCC license issued to your Volunteer Fire Department authorizes the department to operate a specific number of radios on specific frequencies in a specific geographical area for a specific purpose. You, as a volunteer fire fighter, do NOT have an FCC license that covers you on those frequencies. When you are working under the direction of the department, you can utilize
their radios to perform your duties.
FCC Rule 90.427 covers this.
eCFR — Code of Federal Regulations
What it says is:
§90.427 Precautions against unauthorized operation.
(a) Each transmitter shall be so installed and protected that it is not accessible to or capable of operation by persons other than those duly authorized by and under the control of the licensee. Provisions of this part authorizing certain unlicensed persons to operate stations, or authorizing unattended operation of stations in certain circumstances, shall not be construed to change or diminish in any respect the responsibility of station licensees to maintain control over the stations licensed to them (including all transmitter units thereof), or for the proper functioning and operation of those stations and transmitter units in accordance with the terms of the licenses of those stations.
(b) Except for frequencies used in accordance with §90.417, no person shall program into a transmitter frequencies for which the licensee using the transmitter is not authorized.
Again, the part that often gets ignored is that you do not have a license for those frequencies. Only the department does. You do need to have something in writing from your chief, as 902 said, that specifically says that the holder of the license is giving you authorization to have their (and only their) frequencies programmed into your radio. Without that piece of paper, you would be operating outside the FCC rules.
No, the local police isn't likely to throw you in the slammer for that. They aren't even likely to take your radio away, or anything else. But, if push comes to shove, it is an FCC rule and you could be busted for it. Since it is
really easy to just get a letter from your chief, it's a good idea to do that and keep it with you. That will remove any question. Also, have your chief file a copy in your record at the department.
Note that in the FCC rule linked to above, there is no waiver given to volunteer fire fighters, reserve officers, amateur radio operator, friend of the chief, or anyone else. The rule is very clear.
Some more reading, if you feel like it: 90.421
eCFR — Code of Federal Regulations
With GMRS, you shouldn't have an issue. As 902 said, keep a copy of your license with you. Don't use a hand held radio while driving. Basically look at the "distracted driving" laws for your state and follow those.