Yeah that would work just fine. Also, the suggestions made above are great too. It really doesn't matter to be honest when it comes down to doing the job, as long as you are in some type of good physical conditioning. Great fireman come in all shapes, sizes, and strength levels. Strength is certainly always going to help in life of course. If you are able to just perform work for a long period of time, then you will be fine.
A good fireman can come in to work, go fight a house fire, then run medical calls and wrecks for the next 20 hours on a couple meals and very little sleep-- and have a good attitude about it. When it comes to lifting heavy shit you have your crew mates there to help you. No one is going to pick up a 300 pound person by themselves or run into a fire with no one behind them, it isn't like the movies.
If you like to lift weights, then lift weights. If not, do what you like to do to stay in good shape for the job itself.
As far as prepping for an agility test to get into a rookie class, the best thing to do is find out what the test involves and make your own exact version of the test and train it a few days per week so you can pass it with no problems.