Powering up long climbs isn't explosive. It's just power – sustainable power. There's also the weight thing if you are wanting to climb fireroads better – it'll pretty much be power/weight ratio that'll decide how fast you climb.
When you say you are OK endurance wise, I'm guessing you mean you can ride for ages, at a fairly low intensity. For getting up the climbs you'll be riding at a much higher intensity, for a smaller period of time – say up to 30 mins. Limiting factors will be things like muscular endurance, lactate clearance, lactate tolerance.
To get better at these, you'll need to stress yourself in these areas – try doing intervals up a steady climb – say 5 mins on, 5 mins rest x 4, at an intensity that isn't all out but fairly close (say 8/10). Then try increasing the length of the interval.
However, there isn't really a simple answer (though I agree with hels suggestion of lots of hilly miles). Try getting a good book and learning from that – I'd recommend the Joe Friel ones.