nothing wrong with being a policeman or firefighter. These are both positions local cities make good use of.
The problem with government is not really at the local level. Well, it is when corruption gets out of control, mixed with an inflationary currency, where the city ends up like Detroit. BUT if a city ran only on sound money, like gold, there would be far less waste & fraud because people would feel it in their pocketbooks before politicians could get away with it.
That's the problem we have on both a local and national government level (and the national govt level is a gigantic problem, nearing the end by currency destruction).
So you can be a libertarian and be a firefighter without feeling guilty. That's not where your fellow citizens get robbed. Firefighters provide real value, and if you think they don't provide enough value, use your career and stability to educate your neighbors on the benefits of doing away with public firefighting and start your own private firefighting company :) You win win, either way.