If I take a devilfish for my strike team in a combined arms detachment, will the transport have objective secured even after the unit has disembarked? If so, can I disembark the fire warriors on an objective and then take the devilfish to score at another objective?
It does, and you can. You don't even have to start the Strike team in the devilfish, so you could begin the game holding two objectives. A transport bought separately (generally most other transports are Fast Attack) will not have Objective Secured unless it is "Troops" or the mission says it's type has ObSec.
It's worth noting that if you have Pathfinders in a Devilfish and Firewarriors in a second, only one of them will have Objective Secured. The only really confusing situation is if both units disembark and then switch vehicles... Units in a transport do not confer (or revoke) ObSec status to the vehicle.