Having a transportable ladder could be abused to reach unreachable places in the map which would be a serious issue. Sure, that's one of the reasons why admins exist and also reporting, but we wouldn't want people to help their friends go outside the map for afk growing while there are no admins on etc.
On second thought, this could be fixed if it is done like this:
1- Fireman plants the ladder just like a policeman plants the roadspikes. After it is planted it doesn't move.
2-After the fireman plants the ladder he is automatically on the bottom of the ladder, he cannot leave the ladder until he despawns it.
3-While he is on the ladder he can climb up and down the ladder with W and S just like he normally would and at the same time he can put the fire out.
4-When he's done he just despawns the ladder which removes the ladder and puts the fireman to the same place just before he planted the ladder.
Exploiting of the fireman ladder could be prevented this way but even though, I don't think it is necessary to have this. Fires rarely go up to unreachable places, so the time used on implementing this would be a waste. If the fire is unreachable you should just call an admin.
(I don't know why I had to explain that much just to tell I don't support this idea lol)