Jelquing is good to limber up the penis, it will help to break up any plaques that might be lingering and it generally promotes blood flow.
So, yes, it does help.
However the best thing you can do for erectile firmness is Kegel. Whether this is the non-erect Kegels recommended as part of the Newbie Routine, or erect Kegels and their variations, such as the Wally Wally Up, a daily regimen of 50 - 100 Kegels should produce noticeable changes in the firmness of your erection.
As far as length gains and jelquing: it is recommended that one jelq with between a 60% to 80% erection. It generally considered true that if you jelq at the lower and of the erection scale (60%) you will promote length gains, as this is closer to stretching, whereas if you jelq at higher erection levels (80%) you will promote girth gains as this is more of a squeeze.
It goes without saying that if you want greater length ten thirty-second stretches in all directions will be of help.
Always warm-up thoroughly and if at all possible stay warm throughout your routine.
Does that help?
Before: I'd like to show you something I'm very proud of, but you'll have to move real close.
After: I\'d like to show you something I\'m very proud of, but you guys in the front row will have to stand back.
God gave men both a penis and a brain, but unfortunately not enough blood supply to run both at the same time. - Robin Williams (: