Why do firefighters work a 24-hour shift?

Partly tradition, partly economics, and partly the firefighters desire it that way.

Traditionally, the travel time required many decades ago, loss of response during shift changes, etc. led to these long shifts or days "on the job." In addition, firefighting is a lifestyle, not just a job to pay rent. A calling if you will. There are many duties and events they attend. None of which could be done in a regular eight hour shift.

Economically, it is beneficial for communities to do it this way since it cuts down on the number of firefighters required to provide 24 hour response.

Finally, 24 hours on and 48 hours off (if that is the schedule- it varies) is attractive because, baring an emergency disaster, you know you will have two days off of every three. (or work just over two days a week for the same pay of someone else working five days a week)